it is not recommeded for developing girls to wear bras which have under wiring, most wonderbra type garments are underwired so its probably not such a good idea to wear one every day but for a night out now and then shouldn't hurt.
Unless you're pregnant/ breastfeeding your breasts are basically lumps of fat, and pushing fat into different shapes isn't dangerous and they won't stay that way or it would be like wearing control knickers and your stomach staying flat after! If only.
Sorry sft42, I didn't intend to sound unflattering to my fellow womenkind, I was just over-simplifying down to the facts to get the point over. That's not the way I think of breasts really (not that I spend a lot of time thinking about them!).