Actually, for either wheel, it's much easier to take it off with the bike upright, supporting the seat stay or seatpost (for the rear wheel) or the handlebars/stem (for the front wheel) with your other hand, then lay the bike down on its side after the wheel is off. With the bike upright, the rim stays aligned in the brakes, the quick-release stays aligned in the dropouts, and (for the rear wheel) the chain stays aligned in the derailleur and the cassette. Once you turn the bike any way except the way that it's supposed to be ridden, everything gets off kilter and it's much harder to lift the wheel out cleanly.