Hello,

	Personally, I think if it's a reproduction cel,
it should be signed somewhere.  It's customary to
have artists sign their work and it would help
prevent fraudulent resellers from trying to pass
it off as real in the future.  In the past
innocent reproduction cel artists' work have been
used in fraudulent ways by their buyers. 
Preferrably the signature would be hard to remove
both physically and digitally.  One suggested
method has been between the paint and the acetate
(so it's hard to remove physically) and over a
couple tracelines (so it's harder to remove
digitally).  This means you'd have to sign
backwards, but you can always make a stencil or
learn that new skill ^^;   I know it might be easy
for you to tell your own work and for others to
tell that the colors are off or the lines are too
big, but I've seen many people take reproduction
work that wasn't nearly as nice as your own as
real cels.

Keys
PS - I like the ~original~ PJs you gave Utena in 
one of your pieces ^_^