Hmmmmm is it freshwater sand or salt, (live or dead) Freshwater i recomend before, (if the sand is dead) but if it is alive i say after. When you add the water turn off you filter, and make a hole in the bag and slowly dump it in. http://www.youtube.com/watch?v=kxd4ZISTuIM
I prefer before as wel. I take a bowl and pour the water into the bowl so it doesn't disiturb the sand bed too much while filling. Cuts down on clouding. If you buy quality sand and rinse well though clouding isn't an issue anyway.