Personally I say that riding platform pedals without clips is teh best for the knees. Allows you the most float.
I really think that people should not have platforms+clips or clipless until they perfect a smooth technique without the on-off mashing motion. The sudden transition of no-power/all-power is harsh, causes lateral figure-8 wobbling of the knees and forcing your feet to not move in order to contain this motion is ludicrous.
Actually, I think the best method for teaching/learning technique is to ride on a bare pedal-spindle. Remove the pedal body completely (or find some old beach-cruisers at college-campus). When you can exert all your leg force smoothly and in-line with the spindle in the correct direction (tangent to crankarm all the way around), then you're ready for clips or clipless pedals. Ever try sprinting out-of-the-saddle, pushing 100% as hard as you can on a bare pedal-spindle?