Well either way, the risk of sending cash trough
the mail especially to other countries is really
high.  Unfortunately, it seems to come up often
and is unavoidable.
I have sent it both ways, and so far have been
pretty lucky.  
I still would insist on maybe using a envelope
with the security markings inside and/or putting
the money inside folded paper.

I do have to agree with you though japester,
registered mail does stick out like a sore thumb.
 Usually when people see a small registered letter
or envelope they can assume pretty accurately that
it has money or something valuable in it.