Actually, I hope you mean "increases from initial 
value of the CHIPPED cel," not increase in overall
value. ^_- A restored cel will almost always 
remain *below* the undamaged-value base (unless 
we're talking about something exceptionally rare 
and vintage), but can be above OR below initial 
damaged value. Work done by amateurs with e.g. 
nonmatching palettes, of course, will depreciate 
the cel further. 

However, I have heard of some people "repainting"
a chipped cel by melting the chipped pieces of
paint *back onto the cel* - I've never tried it 
and so can never recommend it, it's pretty 
touchy; best tried on junk cels. 

Another thing, if a touch up is really 'required,' 
is to create a correction layer. This applies not 
only to the usual line correction layers, but at 
least with paints, you don't have to worry too 
much about an exact palette match nor any 
potential of ruining the original cel (you still 
have to have the line overlay, though). For the 
majority of people, a correction layer certainly 
does not depreciate a cel in any way as long as 
it remains unstuck from the cel.