I don't think it matters what grit is used; the end of the ring does nothing but hang in open space. The important part is to get the gap right. As far as burrs, the ring should always be dressed on the end after filing, even before each check, so that you're not putting a scratch or groove in the bore as it's pushed down. Some fine grit paper would work for that and would be easier than trying to use a small file. In my opinion, unless you have some experience in using the 2 sided file, you'll have less of a chance of going too far if you only file one end. If you do use one that files both ends, then you'll want to be very careful and sneak up on the correct gap slowly. It's a long and tedious process regardless, using the piston each and every check to make sure the ring is square in the bore, and down the bore a couple of inches so the piston is square and not slightly cocked in the bore. Taking your time will result in a much better result.
