I own a C150, although I dont instruct. I was taught to use 20 degrees for a "normal" landing. Unless you need to correct a high approach or you want to make a short field landing, there is no point in using more - in fact you will just need more power and make more noise. I agree with the point about flap motor burn-outs -- it has happened to me twice. My view is that you should teach students to land on all the available flap settings - flexibility is important - you wouldnt expect a golfer to only learn to use one club. You could also show the more able students how to use sideslip instead of flap -- its fun, and could be valuable in a forced landing with dead electrics.
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