According to Tracy DiSabato-Aust's book (basically a bible for perennial gardeners), if you cut plants by half in early June you will get a later bloom season (beginning mid- to late July and lasting until late September assuming no frost) as well as plants that are about 1 ft. shorter than usual.

^Just to be clear, this applies specifically to Echinaceas; it is not generic info for all perennials.

Thanks!