All the above posters clearly pointed out the merits of using the seatpost clamping method and as a previous postersaid, it's pretty much an industry "best-of" practice. Regardless of the material or bike, we always clamp using the seatpost. It takes a couple extra seconds but we 1). mark seatpost height w/ a piece of electrical tape at the top of the collar 2). Measure & record seatpost height and record it on the repair slip
3) than we can raise or remove the seatpost (for high end posts, we end up using a "dummy" cheap, inexpensive post like a dimension or like or we wrap the seatpost with a shop rag before clamping it)
It takes a couple extra seconds but this way I've never had an issue with dimpled frames, crushed tubes, marred paint/ decals..