Masters coaches, learning bad habits?

I have pretty much been working out on my own, trying to improve through books, videos, GoSwim, Mr. Smooth, you name it. I haven't yet taken the plunge into a Masters class yet, partly because my work schedule is not consistent, and partly because I enjoy swimming on my schedule and doing my own routine. That said, part of me feels that I may be cheating myself of the chance to improve. So a couple weeks ago I was swimming early in the morning and kind of eavesdropping on a Masters coach talking to his swimmers, to see if it might be something I would want to join. Then I asked one of his swimmers afterward how he liked it. "I love it" he said, "I've improved so much. For example, now I always make sure to put my hand in the water thumb first..." It may be that this coach (an out of shape gentleman probably in his 60s, who incidentally I have never seen swim in the pool) will improve many aspects of my stroke, but if there is one thing that I have read lately it is not to internally rotate my arm on the recovery because of the stress it places on the shoulder. So, if I were to join this class do I just politely say "I would prefer not to enter thumb first, but teach me what else you know"? What if he says "OK, pull in an S-shape". I'm sure he knows more about swimming in his little finger than I do, and just having someone watch my body position, etc. would be helpful, but... How do you go about finding the right coach, and do you follow the instructions to a tee? Thanks.