I see those 'on top' corrections done on cels 
that are still sealed in studio bags, so I have 
nothing against it, personally. I'd rather have 
the original look of the cel preserved than brown 
lines. I think it is always appropriate though, 
to 'mention' that a cel has corrected lines on 
top if you intend to sell it, just to make sure 
that the person you are selling it to doesn't 
mind. And, the lines on top 'can' be removed, so 
that the original linework is visible again. But, 
it takes great care to be certain not to remove 
any colored lines that are painted on top. An 
additional layer on top is the least harmful, 
certainly.