I'm with the people suggesting doing line
restoration on a different sheet of acetate. 
Personally, I would much, much rather have a faded
cel than one with lines repainted on the cel, even
if that work is done by a professional.  

Regardless of what a professional may say about
the effect on market value (would they really say
that the work they're trying to sell you will
devalue a cel?), to _me_ that "fixed" cel has less
value.  On the other hand, I would consider having
a good _separate_ correction layer to be a nice
bonus to enhance the cel's display value, much
like a matching laser copy background.

As far as  the "I'll never sell it so it doesn't
matter" philosophy goes, I feel it's unrealistic.
 Things come up and cels get sold, and after a
buyer or two the knowledge that the cel has been
modified probably gets lost.  I've avoided bidding
on a couple of Rose of Versailles cels in the past
 that were sold with no particular comment but had
such good trace lines that they had to be either
"fixed" cels or fakes.  It's a shame, since one
was a scene I'd have loved to own a cel from,
faded or not. :-(

--Atiya

P.S.--I feel differently about the kind of
restoration where professionals fill in paint gaps
by melting the original paint.  This type of
restoration uses no non-original materials and
should leave the cel's appearance identical to the
original.  Of course the fact that the cel has
been restored should still be disclosed if it is sold!