Yeah....tipping is out of control. However, I do tip the mechanics that work on my bike because I want to know I won't get screwed later. It's a small investment for a big return. However, last time I used an In N Out gift card.
As for instructors, I've only had one so far. I was thinking of tipping him until 2 out of 5 students ran out of air on our first deep dive. Then he got upset because the DM questioned why he was bringing students back OOA after hearing us talking.
I'll tip my next instructor if he does a good job. He's teaching me things that could possibly save my life. Hell, I'd have given my drill sergeants $1000 dollars if I knew what their training would mean to me later. That said, my tip will probably consist of a gift card so rather than tipping him, I'm buying him lunch, a couple beers, whatever.