This is one of the areas where I'm finding Android to be so much less pleasant to use than webOS. With my Pre, I always know what apps I have running and can manage them quickly and easily. With Android, not so much.
That said, I've found Android to run just fine without fussing about too much with which apps are running. I do have to be a little conscious at times of what's going on if I have an app I simply WANT to keep running in the background (e.g., Runkeeper to track a walk), but otherwise I've not run into any performance issues that make me want to micromanage running apps.
Battery life is probably a different story, but I'm usually close to a plug of some kind. I'm spending my first time away from home/office since I bought my Epic a two weeks ago, and so maybe I'll feel differently by the time I'm done next year (e.g., tomorrow morning). We'll see...