no need to go all the way down to the wood if you are satisfied with the color of the wood (e.g., you don't want to stain it and their is no problem in the underlying wood you want to address) and the epoxy is in good shape (not cracked, flaking or discolored).

that said, you do need to make sure that you are glassing to epoxy and not varnish and you should monitor the process for color consistency.

by color consistency, i mean if the wood has been exposed to sun it will often change color slightly....and, if in sanding, on some parts you go through the epoxy into the wood  so that now you are into 'fresh wood' you will create a color blotch.   so the whole thing, in this case, may look better to take everything down to that same fresh wood so you have a uniform color/look.

my personal perspective is if it is a strip deck i was redoing and i only had epoxy on it....and it was aged a lot....i would probably just take it down to the bare wood.  i don't think it would be that much more effort.  the nice thing about strip decks is they are often relatively thick and becuase they are not plywood, you can't sand through the veneer and mess it up.  this way i have no doubts about what i am working with.   i would hate to put in all the effort and find out that i did not have a good foundation.