I always do it when ever sending a packet.
In fact, even people at the customs told me to
do so because a painting with a high value may be 
considered an artwork and so need a very complex 
procedure to be imported/exported. But obviously, 
that can't apply to a cel, even a very expensive 
one.

As to the honesty of the things : Well, what's
the actual value of a cel ? Why should you pay a 
lot of taxes because someone else was willing to 
bid a lot on it at that time. When at another 
time, it could have gone for much cheaper ?

If "gift" is a problem to you, you can always
announce it as "sample" : it has the same result.
And after all, cels are really a sample of the 
anime :)

If you send something as goods/produce or if the 
value is high, the customs will have to open the 
packet to check it (in countries with import 
taxes). In that case, that usualy mean that you 
have to pay the taxes but also a fee (which can be 
of $20+) for the work of opening the packet...
Everybody know that post/customs people don't like 
to work so spare them that extra work...

The only thing to remember is that you usualy 
can't have a differant value for insurance and for 
the customs. So if you put a lower value, you will 
also have a lower insurance in case of problem...

Cheers,
Yann Stettler