I don't believe I've ever seen baseboards installed over the edge of the carpet. That would look pretty funny, I think. If the carpet installers are careful (and skilled!), then they will be able to install the carpet right up against the baseboard with no visible gap and no marring. This method requires less precise work from the installers, since they won't need to leave an exactly straight cap along the walls into which the baseboard will fit.
I was in my office when we were getting some carpet replaced and the guy doing it was a real pro. He had a special tool that he used to pull/stretch the carpet with his knees. As he got near a wall, he'd srethch it really right close to the tack strip, pound the carpet into the strip, and then precisely cut off the excess so that the remainder could be tucked evenly against the baseboard. There was never any danger of marring the baseboard.
