Hi, Clare. I agree--if your plants have outgrown their space, you can divide them easily. But also, I have cut some off of my iris leaves when they get really gross-looking. When they get brown on the tips, and start dying, I'll cut them off right under the brown part. I've even cut them at an angle, before, so they look more natural!! This leaves enough leaf to feed the tuber, but doesn't look so bad.
I also clean mine out 2-3 times a year. Irises are self-mulching, which can make them look kind of trashy, so I pull all of the dead leaves out, and I even pull out any leaves that have brown streaks all the way down. I've also discovered diseased tubers by doing this, and saved my irises.
You probably know to dead-head your irises after they bloom. This makes the plant healthier, and it doesn't put so much energy into making seeds
. When an entire stalk is finished blooming, I'll cut it off to the ground.
I agree that they can kind of take over the landscape. But if you can clean them out and trim them a little, you'll enjoy them longer.