The danger is in power-start situations. On an SS or FG bike, you are sprinting away from standstill in a somewhat higher gear than you would on a geared bike (or if not, you're going to be spinning like mad). Also, on an FG, the reversal of forces, when you go from pedalling hard to resisting hard is something you don't get on a freewheel bike. I've got an SS with quick release, and I've never had a problem, but I don't think I'd run skewers on a fix. The only real problem I've had with slippage was riding a road bike with originally had 126 mm spacing, but somebody had fit a 9 speed rig, which meant that whilst it could be sprung into the frame easily enough, the dropouts were no longer parallel with each other. Had to tighten that thing down all to hell to get me across London.