I don't use a power sander for finish work, I find they often leave marks that sanding by hand does not. I do use a power sander for shaping and even for tillering sometimes.
When I'm done tillering and ready to put the finish on, I go over the belly of the bow with 40 or 60 grit sandpaper by hand. This really digs in and gets those tool marks out without you having to put too much effort into it. I try to keep my sanding balanced along the length of the limbs so as not to throw off the tiller. Then I move up to 100 grit, and sand the entire bow, back belly and all. Then I move up to 150 grit, then 220, then 400.
I apply one coat of tung oil, then sand again with the 400, then apply as many coats of tung oil as I need to in order to be satisfied with the look. I've been very happy with my results on the last few bows I've finished with this method.