If you are doing the "cold" browning using LMF Barrel Brown/Degreaser or something similar, remove the sights. The sights will get in the way of the carding of the flats that needs to be done between each application. My preference is to not even install the sights before cold browning/blueing and cut the dovetails for the sights afterward. As for the screw holes, having them rust up to the point of being useless has never been an issue in my experience. If you were really worried about it, you could plug them up with a dab of modeling clay. If you are planning on using the "hot" browning stuff, can't advise on that, don't use it.