Will the watch be inspected before the final price?
A photo estimate is a starting point. The movement, case, crystal, and bracelet should be checked in hand, so the final price reflects what the watch actually needs. You should approve that price before work starts.
What parts will be used?
Ask whether any parts will be replaced, and what kind of parts they are. A clear answer, given before the work, avoids surprises.
What happens to water resistance?
Opening a case affects its gaskets. Ask whether seals will be checked or replaced, and whether the watch can be pressure-tested afterwards. Don't assume a serviced watch is water resistant until you've been told it was tested.
Do you want the case polished?
Polishing removes a little metal and can soften sharp edges. Some owners want a refreshed case; others want every original mark left alone. Say which you prefer before the work starts.
How will it be shipped and insured?
For mail-in service, agree on shipping and insurance before you send anything, and keep your photos and tracking numbers until the watch is back.