I always seal mine with some sort of finish. Antler is porous and will absorb oils and other grudge from its environment. This tends to make the pen's beauty suffer with age. I've seen pens that after a few months use that look absolute horrid and disgusting. Most of my pens, on the other hand, look almost new after several years. The plating usually wears before the finish. I fill all punky open areas with CA. I coat the bark with a finish and buff out.
Other technique I don't recall seeing here is to use a white abrasive when sanding. If any abrasive compound should be released from the substrate, it will cloud or muddy your antler's appearance. Or use micromesh.