It depends on the child, and the severity of the teeth alignment (and the reputation of the dentist!)
My friend's daughter and son had pretty bad teeth problems, to the point where they needed major oral surgery. In their case, it was bad enough that it would have caused problems down the road, no question.
My oldest daughter, who has an overbite (very much like mine) went to the same dentist and he was all like "So, we'll do this, and that, and like 3 rounds of braces, and we can finance it, blah blah blah..."
And my wife, God bless her, basically kept asking the dentist "Is this medically necessary? Will she starve otherwise?"
And after 20 minutes of discussing it, the dentist finally admitted it was strictly cosmetic, wouldn't affect her eating at all. So we told them no. We figured, if something bad happens, it would better to get the surgery done after the fact and get it over with, than have to deal with the added cost and hassle of braces. It's not like she's hideously disfigured, either... this was for a slight overbite.