I've always believed in planting what grows easily in my yard. Not thugs, mind you, that completely overtake the garden beds and have to be rooted out one damn tuberous rhizome at a time (lily of the valley, I'm looking at you). But I do not believe in fussing over my plants. If they can't make due with well-composted soil and an occasional supplemental watering in the dead of summer, I don't need them.
The thing about growing what grows well is that eventually it's going to start muscling in on its neighbors. Then it's time to get the shovel and whack out some clumps, transplanting them elsewhere in the yard. This process is euphemistically called "editing." Other forms of editing include moving a plant to three different places in the yard to find a spot where it does well, yanking out plants that are past their prime, or rearranging the entire planting scheme by digging it up. "Editing" is the gardening equivalent of forced relocation.
Today I edited. CAS put in a new garden last year and would love some pass-alongs. As the iris was slowly disappearing into the hardy geranium currently in bloom, I pulled out the shovel and sawed off a few clumps of the geranium for her. I also moved the white Siberian iris from along the fence (where I'm planning to put herbs) and into some bare spots between the caryopteris in the garage blue-and-yellow border. They'll be almost completely engulfed by the caryopteris when it leafs out, but that won't be until later in the summer, well past the irises' bloom time. In the process I divided those. I heaved the clumps on the ground, put the point of the shovel in the middle of the root mass, and jumped on it. Voila! Multiple iris plants. And I'm pretty sure they'll do just fine, even though they weren't divided in the fall.
Many will say you should only divide perennials when they are resting: in the spring for summer- and fall-blooming plants (such as asters or coneflowers) and fall for iris and geranium. I say that's all well and good, and it does improve your chances of successful transplant. But my babies are all pretty hardy and unlikely to take offense at an out-of-season move, provided I keep them watered. Besides, I like the instant gratification of moving plants when I get an inspiration. Also, I had to jump on an shovel jammed into the root mass to separate the iris. It's not a fussy plant.
I'm just saying. Plant what grows well. It cuts down on headaches, you'll be able to take lots of divisions and give them to your friends, and you'll become very popular at the garden club meeting.