from my perspective.
The purpose behind sanding Urethane clear is only to remove imperfections and any texture, ie orange peel that you may have obtained when spraying. Depending upon how minor or major the imprefections are , I personally start with 1500 to remove the major portion of the texture It is important to use some sort of pad when sanding to reduce the pressure points, and also if you dont use a pad you wont eliminate the texture completely without sanding excess material off. When I am removing something significant, say a sag, or a dirt speck, I have even used a short peice of paint stick as a backer for the paper to insure that you are cutting it flat. After removing the majority of the imperfections with the 1500, (or 1200 if they are significant in nature) then fiinal cut with 2000 grit. this will minimize the amount of polishing you need to do to get rid of the sanding scratchs. As you are sanding, periodically clean the surface, if you are doing it right, you will be sanding the tops off the texture and the valleys will still be shiney. Depending upon how perfect of a job you are after, and how much clear you have to work with, determines when you stop sanding.
Phil's article was VERY informative and is a must read.