Again the protection from wear is looking many years ahead in the life of the bass. I've never done it but I'd suspect there is risk of screwing with the poly trying to install frets after it's applied. Even cleaning with the fret saw might chip it. Putting it right over the frets then filing/sanding off sounds easier and less risky. If your not grain filling the fretboard you'll have visual contrast anyway which is why I'd just oil it, plus I think it would look cool and highlight the super-long fingerboardI considered that. That would be the easiest and it's still an option. The 3 reasons I decided to poly the whole thing were, it would all look consistent, it would be simple and it would offer a little more wear protection in the spots where my thumb would contact the board and pup cover.
But when I get to the part of poly before or after frets, the simple part goes away.
Another thing I'm not sure of, is if having poly down first would cause any problems with seating the frets. I would assume it wouldn't, since even 3 coats of cured poly is so thin. But I don't know for sure.

