By the Fourth of July, you can and should prune your perennials to keep them looking good and performing as long as possible. In England, this is called the Chelsea Chop, and it coincides with the Chelsea Garden Show in May. Our season starts much later than England’s, so we do the chop by the 4th of July. Since the biggest flush of blooms occurs in May and June, July works well as a mid season clean up time.
I don’t think sedums such as Matrona or Autumn Joy respond well to this kind of pruning, so I leave them alone. But I do clean up roses, daylilies, alliums, daisies, lavender (if it is finished blooming), dahlias (disbud for larger blooms), lilacs, forsythia, echinaceas, heleniumsm, Joe Pye weed and phlox.
If the idea of whacking back your perennial/herbaceous borders makes you shake, read up on it in The Well Tended Perennial Garden, by plant wizard, Tracy DiSabato-Aust. The book is so good, I have recommended it for YEARS. It’s been updated again, so get the Third Edition.