Personal preference. I like a little bit of float, not because of any knee problems, but just because I like the feel somehow. It's whatever feels right for you. If you're not sure what you want, one good option can be to get pedals with adjustable float... try different floats and fiddle with it until you get what you like. As one example, Speedplay Zero pedals have pedals with adjustable float... I think the range is something like 0 to 15 degrees.