Quick chime in. I've been using instructors for years both ski and flyfish. I view tipping as optional, but I budget for it as if I'm going to do it as part of the vacation budget. So if I know going into a trip a rate is $500 or $1000, I immediately add in the assumption of at least 10%-20% and then sometimes I have to back out of that with the question: is actually hiring a guide/instructor within our travel budget. We love instructors but on some trips we don't add them.
I have always tipped because almost every instructor or guide we've used has been good to great. The way the industry "often" works in both ski and fish is the booking agent (i.e. the resort or fly shop, etc) often takes at least 40% upwards to 80%. (In fly fishing, the lower rates are usually reputable high volume shops that expect the guide to meet the clients at the fly shop to stock up on gear.) But whatever the quote rate is it's likely the majority of that goes to the agent, not the guild. So unless a guide/coach is horrible, we minimum 10% just to pad the hourly base rate they get and that's for folks who just do the basic stuff, but I try to go in assuming I budget 20%+ because I want to be psychologically ready to reward for awesomeness in advance.