The recommendation that I've always heard is to pinch them back in the spring, but no later than July 4 (in order words, stop pinching by that time). Then they will bloom in the fall.

When you buy mums in the store, what you see are plants that have been pinched all through the season (and that's how they get so bushy also). They are being forced to bloom later than they would've in nature.

You may cut them back now and they will bush out again and bloom in fall if you wish. Just give them a bit of fertilizer after they begin to grow again and keep pinching. 4th of July has always been a bit too early for me(it's fine for further north) I pinch til August for good fall color. It keeps them from burning in August's hot sun and extends blooms til close to frost.