I haven't been collecting cels long, but I have 
been an anime fan for most of my life (about 18 
years). The distinction for me between Hentai and 
cels like the one Tavington is selling is in its 
intent and not its content. Hentai is an anime of 
an overt sexual nature. It's main purpose is 
arousal. In the case of nudity such as in Tenchi, 
it is more story driven and used as a fan 
service. I would not seperate nude cels from 
others cels unless it is of an overt sexual 
nature (i.e. two characters stripping down their 
gear and getting it on). Now, if you want to 
cover it up because you don't think that kids who 
are going to view your gallery will get this 
distinction, then by all means do so. Hope this 
helps.