Yes, I think a warning of "No one under 18" (as 
if that'll work) is proper.  It's okay to put it 
on in Japan, as you said, but in the USA I find 
it's better to cover it up or warn people.  If 
you pay a lot for a cel and are proud of it, you 
will not want to cover it up, so a warning should 
be good enough. Yet, other people will say show 
it all and don't put any warnings there.  We all 
have our own feelings on this.  I am a female and 
I HATE movies full of nudity cause it's all nude 
women.  I'm sure the men love it but females do 
not get into that stuff.  So, it's no fair once 
again with anime since it's always nude women and 
once again we females have nothing to look at.  
But, this does not seem to bother most people.  
I'm just old fashioned, sorry.