I always sand between coats. The solvent in polyurethane (mineral spirits) doesn't partially dissolve or melt into the already applied coat like a second coat of lacquer does, so it supposedly helps to make small sanding scratches for it to bond to. Whether that is true, or really necessary, I'm not sure as I've recently read that it is just a myth and the second coat will bond just fine without the scratches. What I do know, is no matter how carefully I apply the coat of finish, there is usually something that isn't quite smooth, whether it's dust, bubbles, bugs or something else and in a slow drying finish like polyurethane, there is always something. I sand to smooth out all those imperfections between coats. I have used several spray/rattle cans of poly.