I agree with everyone else--we have a bunch of 
Slayers cels, and only one has line fading (it 
happens to be a zelgadis, but I think it's more of
 a reflection of how the cel was treated before I 
bought it).  Some of my early season cels look 
just as good as later ones...and many of my 
Slayers (and Lost Universe) cels are the 
better-looking (in terms of crispness and lines) 
that I own.  I think the 'history' 
of a cel is the most influential factor in fading.
In most of the series I collect, I have some 
cels that are faded and others that look great.  
El Hazard TV (the Wanderers) seems to be the worst 
of the series I collect in terms of line fading...

:)rosemary