I do this too but generally don't switch unless the flight is really early in the morning and I want to try and sleep. I've never been turned down. Last time the person I switched with slept all curled up in the aisle seat so the area was clear in front of her seat and I could get up as I wished without disturbing her. I think it is perfectly acceptable to do this and agree that this is the best strategy when you are both wait listed for upgrades. That way no one gets stuck with the middle.