Even with a lot of float, cleat position DOES matter, because when the cleat is off-center, you're still putting a little torque on your knee.
Some people's knees are actually happier without float. My DH and I both fall into that category. He actually ditched his Speedplays and went to Look because the float was irritating his knees. I can ride the grey (4.5°) Look cleats, but I prefer the black ones (0°), and either way they have to be positioned just right or my knees aren't happy.
Speed comes from what you put behind you. - Judi Ketteler