Are you talking about the clear and gold stickers 
that come on some of the cels in the lower right 
corner? If so, that is a seal from Studio Perriot 
(ignore my bad spelling) and I think it is for 
their 25th (?) anniversary. I have a Fushigi 
Yuugi OVA cel with that seal - it isn't just 
Tenshi ni Narumon.

I don't think it makes the cel worth more or less 
except that the cel would be worth less if you 
damaged it while trying to get the sticker off. 
If you really want to remove it, I would 
recommend using the same ways prescribed for 
removing tape - namely, remove the sticker itself 
first and then remove the stickiness with other 
pieces of tape.

Me, I wouldn't remove it (and haven't from mine). 
But your milage may vary.

Many Sharp Smiles,
--Drac