If your cleats are on perfectly and you have a perfect pedal stroke, fixed. If you flap your feet around, then you need some float to save your knees (float is 'yaw' of your foot). Difficult to say, but you can ride ones with float without flapping your feet around..
Good advice above. Don't mess with your knees. Once they're gone they're gone. I've always used yellow, although technically I have a very good pedal stroke. Not worth the risk IMHO.
I've never seen the issue with having a bit of float, fixed doesn't offer any benefit really and as said you have everything to lose if your knee's get damaged!
Very rarely are people so well set up that they don't require a bit of float, and if you're fixed and its not 100% then eventually your knee's will complain, ankle never usually an issue, I've had knee pain, but never ankle or anything other.
