I would definitely recommend the acetate layer if you ever 
plan to sell them.  I know that I am probably in the 
minority, but I don't buy cels that have been signed unless
it was a special release or something.  I just think it mars
up the cel, but I know most other people think it raises the
value.

Also, while the I might not be so turned off by the idea of
a signature by the character designer, I would NEVER buy one
that had been signed by the Japanese voice actor.  If the 
anime has been dubbed and it's been signed by the American
voice actor, I'd say that then the only thing it's good for 
is a large coaster for beer mugs, heh...

Anyway, the truth is hopefully you are collecting for 
yourself and not in order to sell them later, in which case
you should do whatever you want and not worry about it.
Besides, there will always be so many different opinions on 
this type of stuff...