I agree, they are being paid for knowledge and effort. I don't tip
my physician or lawyer or car mechanic because I'm being billed for
services rendered.

However, I do bring krispy kreme or brownies every once in a while.
Makes everyone in the shop happier. And they're more likely to
talk to me immediately when I come in with a problem. The actual
work may not be scheduled for a while but it's never "I'm busy
let's talk about this in 10 days..."