I certainly think that payment for any sort of labor should be customary. The fact that mechanics or shops can throw in freebie labor to earn goodwill shouldn't lead to an expectation of this. It's not like bike mechanics are earning investment banker fees; every little job matters in terms of the shop keeping the lights on and the workers making their rents.
On the other hand, negotiating labor costs after the fact is kind of silly and is probably what led to this whole thread. The buyer simply clarifying, "How much will that be?" before the wrench lays hands on your bike will reduce stress for everyone involved. Then it would be known if the charge would be $10 for a successful job or $60/hour or whatever else. Of course, the wrench volunteering a price before starting the work would do the same thing and require fewer people to change their habits.