I remember something a mentor once said to me several years ago, in a
conversation about human social touches: that an ancient philosopher
(Greek or Roman) wrote that the last place on another person that one
touches is the top of the head.  I have since forgotten the name of
this philosopher and am interested in finding out who it was, but I am
at a loss.  Also if it's not too much trouble, I'd be interested to
know the name of the treatise in which this was stated.

Were there no hairdressers in those days?

It's hard to see what this would have to do with classical philosophy;
perhaps that's why this chap is obscure. Could you possibly be
thinking of the belief of Buddhists, especially in Thailand, that it
is rude to touch someone on the head. because the head is considered
to be the most worthy part of the body?

Just to say that I agree with Geof:
1)  don't know of any classical philosopher's opinions on this;
2)  in Western culture, there is no problem with patting a child on its head;
2)  guide books for visiting Buddhist countries do tell one not to do this for
    reason mentioned.