He said this in a jocular tone, and the whole situation didn't feel awkward at all, but it made me wonder: How common is it to tip a plumber? I would have thought: Not at all.
The answer to that question obviously varies from city to city, so please state where you're from. Me, I'm from Vienna, Austria.
EDIT: I asked around: Apparently it *is* common to tip plumbers here. Like any kind of craftsman. But only after they have finished their job and not too much. (About 5 or 10 € as a rule of thumb, no matter what the bill is.)
And (amongst my coworkers at least) there's a tendency not to tip craftsmen any longer, as they usually get paid *very* decently anyway.