fwiw, i always sand the tape where i can reach it....with my hand carefully protected by a sanding block.

the main reason for doing this is to knock down the edges of the tape which are often sharp and can therefore snag or damage gear that i may store in the boat or otherwise cut a hand when the hand for whatever reason is inside there (e.g., loading the boat or adjusting a foot pedal).

i also do it to make the boat easier to clean and dry.  edges that can form on the tape trap sand and possibly water as well.

on varnishing, i don't usually apply the same level of dligence/scrutiny on the inside as i do on the outside and at least for me, i don't varnish anywhere the sun doesn't shine.   if the sun doesn't shine on it, there is nothing to protect and i may also need to refinish or repair at some point and varnish then would have to be removed to have an effective bond...so i don't put varnish where it is not needed.

I did a rough feathering of the tape with 80 grit to knock it flat, followed by a light sand after a fill coat on the cockpit floor.  Then we primed and painted the whole interior with Interlux Pre-Kote and Brightside to make it easy to clean and UV stable. 

Don't forget to smooth the underside of the deck tapes too.

Paint the interior before joining the hull and deck but leave an unpainted 2-3" strip around the edges for the hull/deck seam tape. It's easy to come back after and sand, prime and paint the join. 

I appreciate all your feedback.  I shall sand the edges where I can reach.