If you have a good relationship with him, don't know why he'd be offended if you wanted to do some rasping between visits. I understand he may be looking at it from an economics point of view, that just seems a bit short-sighted to me.
If he's concerned about you "ruining" them, he should be willing to coach you as to the correct way. If, after a couple of visits, he finds you are creating issues he needs to correct, then would be the time for him to recommend you not do it yourself. In my opinion, of course. ;-)