I've done it a bunch of times. It seems like the guy at your paint store doesn't know what he's talking about. I wouldn't sand/buff a metallic or pearl unless I absolutely had to, but I've never had a problem with solids. Personally, I would put on 4 coats. A good rule of thumb is to put on one extra coat if you're going to sand and polish. A typical BC/CC job has 2 coats of clear so I put on a third to give a little extra material to sand off. Usually a SS finish has 3 coats so I give it a fourth. I guess it also depends on your particular color and how well it covers. Just give it a little extra flash time in between coats. With the extra material comes extra chance of trapping solvents.