I have found it to be much more satisfactory to precoat with epoxy before doing the filling. If bare wood is filled and because I cannot match the wood exactly, every place I smear the epoxy filler leaves a blotch that is very difficult to completly sand off if the bare wood is filled. If the epoxy coat is on first, it keeps the filler from getting in the wood pores. As to sanding it is very easy to use a scraper and scrape down the filler patch without going through the epoxy precoat. Also with a precoat I can see the shade of the finished wood more closely and get a better match.
Earl