All I usually do is erase the gray background 
and make it white to match the background of my 
site, so all you see is the cel image and not 
the ugly dark scanned clear acetate.

A couple times there was dust everywhere, making 
a dark section of the cel look really speckled 
like you mentioned, and I cleaned that up.

I see no reason to mention anything like that, 
as its not part of the cel.  But I usually post 
the original huge untouched scan when I am 
selling a cel anyway, and make note of anything 
that is wrong with it.

:)