I always retouch my scans.  This includes 
cleaning out dust specks, restoring paint damage, 
particularly in hair tips, and to some extent 
correcting the little "cuts" in trace lines that 
seem to be part of the painting process.  

If someone expresses interest in trading for a 
cel of mine, then I send them the unretouched 
scan (which I keep on another computer) and come 
clean about any damage.  But since my cels 
generally are NFS, then I feel no more bashful 
about touching up flaws in my cels than I do 
about recycling layers from one cel set (as I did 
with my CCS cels of Sakura capturing The Fly) to 
image other cels that did not come with the top 
layers.

In my mind, it's the difference between a "for 
sale" gallery and a "come and enjoy" gallery.  
And since the images were obviously cleaned up in 
the process of making the animes from them (as I 
can tell by comparing the cels to the screen 
captures from my DVDs) then I don't see that I'm 
doing anything that hasn't been done before.