I'm plastering my first drywall and have been given advice from a friend who as done it a few times before. He believes I should not sand down the mud (a.k.a. jointing compound) between coats, but I think I should. I don't think the first layer is as smooth as it should be and I think getting any little bumps out now before the next coat would be a good idea. Surely the next swipe of the knife is going to catch on them and leave elongated ridges or scratches down stream.
Yes, knock off any bumps between coats, but there's no need to get it perfect. A screen sander on the end of a pole is the best tool for this job. And it goes without saying that you should minimize any bumps while the mud is still drying to avoid having to sand it later. Some even recommend using the premix joint compound on the last coat and, just before it dries, going over it with a slightly damp sponge to take off any bumps.