I do that and I get very good replies 99% of the time, even from the people who I know we screwed things up for. They don't realise what happened because it got sorted out before they found out, or because someone took a risk that something wouldn't be needed and they got lucky that it wasn't.
Its hard to explain this without saying what the business is, but put it this way, we are the experts and our clients don't know what to expect so they are just grateful that we've looked after them, which mostly we do, but sometimes not as well as we should.
There are a few people who always complain too, even if you really have done the job in an exemplary manner. I was shocked the first time I came across this - the motivation was to get a discount after the service had been consumed.
This is all by the by. If Joseph expands, he'll have the same problems. Most people will say it was great and their number could easily include the ones whose boiler appears fine but it isn't really or for whom the plumber replaced more parts than were necessary.