Now is the time of year to make the adjustments. Much less painless than a root canal, but just as important. This is where the plant gives you a dividend, not unlike a two for one or three for one stock split. This is the truly inexpensive way to populate your garden with your favorites. Generally, perennials won't need dividing for three or four years following introduction to your garden.
Some plants just get bigger and bigger in size.
Others seem to expand from the center out. In a few years, you have a dead or ugly center and the young vital parts are in a circle around the center. Asters, astilbes and chrysanthemums are three examples. Don't bother to compost these tough centers because they take forever to break down. Don't be nervous about this.