If it makes no difference, then it's not a concern.
You didn't answer the other question of whether or not unrecognized fields are read into the database. That could greatly affect the size of the database if, for example, someone has extensive lyrics or comment text embedded in their files.
Other questions:
Does Rockbox utilize sort tags such as ARTISTSORT (TSOP in ID3v2) or ALBUMARTISTSORT (TSO2) for sorting the artist list? From what I've seen to date, it does not.
If not, is there some way to have Rockbox strip leading articles from artist name, so that something like 'The Rolling Stones' appears under R?
When browsing albums, is there any way to have them ordered by something other than album name? Such as artist/year/album?