Bikes come from the factory with the pedals, seat and front wheel off. Some of them come in more pieces than that. The shop mechanics have to finish assembling the bike before it can go onto the sales floor.
Assembly is a seasonal ritual. Shops receive preseason orders in the fall or early spring. Depending on the shop's off-season business, mechanics will assemble these bikes over the winter or in the spring.
Faced with a large number of assemblies, and perhaps limited time in which to get them done, one might be tempted to take the shortest route and simply slap on the parts that aren't attached. If you have limited mechanical staff in assembly season, but more and better mechanics when you're actually selling, you might get away with that. Or, if you think your customers won't know the difference and don't require or deserve a bike that's put together well, you can justify taking every short cut.
At the most basic ethical level, the handlebars need to be tight and the brakes have to work. Anything else is gravy. But you leap quickly into a large gray area if the customer actually rides the bike much.
I prefer a more detailed approach. It is better for numerous reasons. You don't know who will buy the bike or how busy the shop will be at the time. A well-assembled bike is less likely to fail prematurely. Breakdowns embarrass the shop as well as possibly imperiling the customer. Why take the chance?
How detailed is detailed enough? Rather than waste time wondering whether this is the time you need to do a good job, why not make a habit of doing a good job?
Grease the seatpost and pedal threads before installing each of those items. If the bike has a quill-type stem, grease that, including the wedge that holds it in the steerer tube.
Before installing the handlebars, make any adjustments to cable routing, such as crossing the shift cables for smoother operation.
Remove the rear wheel from the frame. Remove the cassette or freewheel from the hub. Adjust the hub cones. Start by locking the cone and locknut on the right side, which will be concealed inside the freehub or freewheel body. The right side is more likely to stay properly locked after that, so you can do future adjustments from the accessible left side.
Grease the threads of the cassette lock ring or the freewheel before reinstalling the gear cluster. Set the rear wheel by the truing stand.
Adjust the cones of the front hub. You may have to pop the little rubber dust covers off to get to the locknuts. Just do it.
Before truing the wheels, grip pairs of spokes in your hands and squeeze them as you work your way around the wheel. You'll hear a crackling noise as accumulated stress is released from the spokes. You can perform this procedure with the wheel in the truing stand. You will probably notice that the wheel goes out of true as a result. Give everything another good squeeze and then true the wheel.
With both wheels true, put air in the tires, install the wheels and adjust the brakes. On linear pull brakes it's a good idea to undo the nuts on the threaded posts on the brake pads, grease those threads and reassemble before adjusting pad height and angle. It's a good idea to grease the threads of just about anything with threads.
For linear pull and cantilever brakes, set the pads high on the rim, near the outer edge, so they stay on the braking surface as they wear. On brakes with pivots beside and below the rim, brake pads hit lower as they wear. If they're too low at the start, they will quickly drop below the rim and develop a lip.
For caliper brakes with the pivot bolt centered above the tire, more common on road bikes, the arms come in and up. Set the pads nearer the bottom of the rim.
To eliminate squealing brakes, you may toe the pads slightly so that the end nearer the front of the bike hits slightly sooner. This is more crucial on cheap brakes made of soft metals that deform a lot under braking force.
As stated above, grease the threads of just about everything with threads. This includes water bottle and rack bolts even if no accessory is installed, and the threaded adjusters of brake levers, shifters and derailleurs.
Greased accessory bolts save you that step if you're slapping accessories on a newly-sold bike on an insanely busy day. On the same insanely busy day, well-assembled bikes take far less time to check over with a customer breathing down your neck. Later, when they come back for the free tuneup, a well assembled bike will need virtually nothing unless the customer has damaged it. That's more time saved when impatient people might be standing around tapping their foot.
Greased adjusters work more smoothly and don't corrode as readily if the bike gets used hard or stored carelessly.
Grease the crank arm bolts. They will torque more accurately. Grease the top cap bolt of a threadless headset. Ungreased, those bolts can rust into the star nut.
Why isn't this the bike owner's responsibility? It certainly could be. But most people don't have the time, knowledge or confidence to maintain their own bikes. They depend on the guys at the shop to be trustworthy, courteous and kind, as well as knowledgeable and proud enough of their work to do it well. You're stuck there for hours anyway. Why not do a decent job?
Ironically, shops make less money on expensive, high-end bikes, so they may be more tempted to cut corners on assembly. But if a mechanic follows a standard, high-quality procedure on every bike, he or she knows what's called for and just does it. Speed comes from efficiency, not from leaving stuff out.
It's difficult when an employee has higher standards than the shop that employs him. The shop management then views meticulousness as a liability, or as a quirk they indulge until they lose patience. A shop's reputation depends on what people think of it, not on what they really produce. So friendly goof-ups will have a bigger following than crusty perfectionists. But which bike would you rather be riding down a hill at 20-40 miles per hour?
I confess I do not take apart the cranksets and grease the chain ring bolts unless I'm doing a complete overhaul. Some things are beyond the scope of an ethical assembly. I do it on expensive special-order bikes, but not on routine assemblies. I can justify everything else I do by showing advantages to the shop as well as the consumer. So a word to those who care: grease your chainring bolts.
Even though cycling is a trivial backwater occupied by the dregs of society, I try to imagine what we do matters. Someone truly important might, for reasons of temporary insanity or gross misjudgment, briefly take up cycling in youth or adulthood. In that case we would hold this person's destiny in our greasy hands. Imagine uncaring slackers building and maintaining commercial aircraft, or the highway bridges over which millions of people travel every day. Cynics will say that they already do. I say that in any occupation, more pride in craftsmanship can't hurt.