THE most unlikely source pointed me to this website which compares "the bounce" allowed by various brands of sports bras. (Maybe it's not unlikely at all- maybe it's a guy thing.)
Anyway, the "correct" foundations are critical to support and comfort during much of what we do. Think running, double unders, box jumps. LIttle to no support can be painful and embarrassing in the short term, and can lead to stretching of the Coopers ligaments over the long haul.
So HerRoom.com has videos showing some of the most popular sports bras in action. This might be a good starting place if you're looking for some new support for your girls.