Its an interesting question and the answer should be its country dependent.
I always find it hard to judge this, because in every country the tip amount is considered differently. For example to my knowledge in Japan you should not tip, they have a very different service mentality.
Personally I believe salary should be such high, that there is no tip needed. It should be reserved for exceptional help/information and so on. But I get one should "comply" with the country standards. I look for info before I get there, and include this in my "calculation" if I find this price is fair.
Back about 12 years on the US I was advices to give 5 US$ each dive during a course, which I gave afterwards.
So the question itself is a good one to me, but the answer is not clear to me.