On Factory original paint, I would only wet sand a complete vehicle, if I absolutely had to. Sometimes it is very difficult to get all of the sanding marks out of the factory paint. On repaints, within 24-48 hours is the best time to wetsand and buff. The paint works up really easy in this time frame.
That said ....
I usually will work out scratches in the finish by wetsanding with 2000 grit paper. I have found that I can control how much paint is being removed and in which area, easier than by just buffing with a more abrasive compound. When you do wet sand a scratch or any other defect you have to blend it in somewhat. What happens is that you are removing some or all of the orange peel of the paint and you can end up with an area that is perfectly smooth right against an area that has a lot of orange peel. This many times looks worse than the defect that you removed from the paint.
I haven't used Meguiar's uni-grit paper, but have heard many good reports about its quality. I usually use 3-M's wet and dry paper ... available at most auto body supply stores.
