Any bike other than a gas pipe special - I never clamp a frame tube. Never. Good way to ruin things. Lot easier to replace a seatpost than a frame tube or a complete frame.

As for a rag, it allows me to clamp the Brommie seatpost a little less than if it weren't there, thereby (hopefully) exercising less pressure on the seatpost itself as well as preventing injury from the clamp jaws.