I don't mean to throw a wrench into all of this, but thought I'd share my experiences with tipping: As a new diver I was grateful for the time and experiences shared by my instructor(s), and tipped them, bought beer, or occasionally both - it depended on how well "connected" I felt to them, and how much I learned from them, and enjoyed their company.
An exception to this was when I did my wreck diving cert with Elena (Mselenaous), and got to spend an afternoon wandering around in the dark with Pete (NetDoc); during our return to the mooring on the Spiegle Grove I started coughing, and went through about 1250# of air before I could stop. Elena was kind enough to share air with me during our ascent, and when we got close to the surface I went back on my own air. I treated Elena to lunch, and I so enjoyed her and Pete's company, that I invited them to dinner that night (with my wife and I); I not only considered this money well spent, but also an investment in a friendship.
Diving in Mexico, a $5 per tank for (local) boat crew seemed acceptable, yet I still like to tip at least $10 per tank; in Cape Town, SA, I was surprised to see a group of French women not tip the DM/boat crew at all (we did), and when I asked the DM (lead instructor for the shop), he said that is normal for a lot of the Europeans.
As for tipping instructors, when my wife finally decided to go for her certification, I made sure to show my appreciation to the DM and instructor candidates - my wife (at age 53) was the only adult in the class, and as someone not fond of getting her hair or face wet, she needed a little extra coaching. I felt that these guys had been very patient, and done an exceptional job, and so I wanted to reward them.
As you can see, it all comes down to local practices, comfort level and how well they did; as a hopeful future DM, I like the idea of paying it forward!
KevinL