Oh my. Okay, true story. **Yours truly** worked in the lingerie department during college for a major department store. I was trained to fit bras. Okay, that consisted of an hour of an old lady telling me how to do it. I sincerely doubt the bra person at your store knows what they are talking about. If anything, a tight hold will keep you from sagging when you are older. :>) I wear sports bras and regular bras. The sport bra keeps me from moving all sorts of ways. Your nipples have a life time ahead of you to go through changes. Wait til you have kids! Holy nipple change!! They get bigger if you nurse. Anyway, long story short, this sounds like rubbish and would not worry about it.
Your growth? Or your nipples' ability to contract and stick out? My only complaint about the sport bra per se is that usually if my nipples happen to be sticking out, they find a way to go sideways because the front of the bra is so tight. But I don't think that is a long-lasting effect.
Ask your ob/gyn. Probably the lady at the bra store does not have a Ph.D. in anatomy.