As noted above, there are a few different ways to accomplish a leak check.
Soapy water sprayed on the reg is probably the least reliable.
A wet test is the quickest approach, and a 6 or 13 cu ft tank makes this a lot easier.
Pressurizing the reg, then closing the valve and seeing what happens to the pressure reading on the SPG is effective.
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I often see customers come in for reg service just prior to a trip and there are some risks involved.
1. These are often the same customers who have not touched the reg since the last trip - and I have encountered divers who dunked the reg in the rinse bucket without the dust cover on the reg with the result the reg is badly corroded and I can't get all the parts needed to repair the reg in the time frame available.
2. Good bench testing and wet testing will catch 99% of the problems and catch them 99% of the time, but nothing beats a couple of test dives. Still, it is not unheard of for a reg to develop problems on the second or third dive after an annual service. In some cases this may be a slight freeflow due to a tech failing to adequately anticipate the initial seating groove that forms in a low pressure seat (something that just requires a simple 2 minute adjustment). Much more rarely, it could be a more serious issue related to the failure of a new/defective seat or part. From time to time manufacturers have QA or design errors and those are often detected when problems occur in the field.
3. Qualified techs vary in their skill levels, both over all and with specific regs. Experience is the best teacher, but unfortunately from time to time that experience is gained through failures. I have had one serious failure on a reg I serviced and that occurred a few years ago with a 1970's vintage R109 second stage. Over the 30 year life of the reg, the arms on on the lever had gotten weak, allowing the tabs to slip off the poppet. It bench tested well and wet tested properly, but failed about 40 minutes into the first dive the customer did with it. I had serviced hundreds of them to that point in time and never encountered that issue before - or since, yet it is still something I check for on the regs I service.
4. Mecahnical parts also tend to get comfortable in their orientation to one another, and some problems related to excessive wear can be aggravated by a rebuild. As a non-scuba example, the majority of aircraft engine and airframe issues I experienced all occurred just after the aircraft came out of maintenence, due to either errors or omissions in the maintence or due to new parts not properly breaking in during the first few hours after an overhaul, etc.
5. Customer preferences come into play as well. If you want peak performance in terms of inhalation effort on your second stage, that tuning is a bit more sensitive and may result in a slight freeflow as the seat breaks in. If you mail me the reg for service or if you are are going on a trip, the best you are going to get in terms of performance is smack in the middle of the (now very common CE detuned) factory specs. It protects me from a customer complaining about a leaking reg and it prevents you from having to have adjustments made to the reg on a trip.
All of the above argue for having your reg serviced at the end of the a trip, or at least far enough in advance that you'll be able to do a couple warm up dives with it before taking your next major trip.
If a diver comes in and wants a reg serviced prior to a trip, provided the service interval has not been excessive and the reg is otherwise clean and working fine, I will offer to bench test it for free and then have him come back in to get it serviced after the trip.
