I gently clamp the top tube with just enough grip so it doesn't fall off. The clamp has rubber on it and frankly I've never worried about it, if you can grab and carry the bike by the top tube, gently clamping it isn't any different. That's good enough for cleaning and fine adjustments so on.
If I need to apply some force to the bikes, like removing the crankset, hammering out the bottom bracket or so on, then I set it on the floor.