If you look around, perennial gardeners fall into a couple categories. Some like their gardens neat and orderly (not necessarily formal), others like the natural look, like a native prairie. I personally try to keep it neat, but sometimes things just get away from me. A few years ago, I bought a book called "The Well-Tended Perennial Garden" by Tracy DiSabato-Aust. In it, she advocates cutting back certain perennials early in the season; for one, so that they don't get leggy and floppy, and secondly, they should flower more, albeit later. I have used some of her recommendations and it works very well, to keep things sort of on the clean side. I like the fact that some things bloom later, because it always seems like there is a time in late summer where perennials get a little ragged and unwieldy. She provides descriptions of certain plants that benefit from pruning. Similar to mums, which have always followed the "pinch back by July 4" rule, I've used it for sedum, coneflowers, summer phlox, balloon flowers and heliopsis to name a few. I highly recommend this book and refer to it often.