I have both bagged and unbagged cels in my 
book.  None of my unbagged ones have ever stuck, 
and they've been there almost a year now.  
However, I do live in a fairly cool area where 
it doesn't get particularly hot, so there's no 
melting.

I suppose I would suggest doing it, though I've 
had situations where the actual cel bags 
took paint off one of my cels. 9_9

-Chaotic Serenity