Fresh paint (i.e. less than several MONTHS old) is easily damaged by clamps, even with padding. I've seen many a frame with cloth texture pressed into the paint from clamping on a rag wrapped around the tube. And decals are easily destroyed by clamps.

Always clamp on a seat post to avoid these issues. If for some reason the clamp won't fit on the exposed post (non-round "aero" style, bolted on accessories, etc.), get a cheap steel post of the proper diameter and insert that for the clamp to grip when you work on the bike. If you work on lots of bikes with different diameter posts, get one of the internal clamps mentioned above.