Man4 I am moderately touch averse, and am acutely aware that other people will probably find that weird, so would never say anything if someone is quite 'touchy', I would just avoid them in future. If I am not mentally prepared for it then unexpected touch will usually make me flinch, and most people pick up on that, some definitely don't though.
Having said that, I say just be yourself, but be watchful for a reaction or lack of reciprocity, and ask at that point. Virtually no-one will say 'back off' if asked, you will have to guage from enthusiasm of reply.
I do know that I am fine with touchy people if I think there is some degree of acknowledgement, however if I don't like them for some other reason then the touching is weapons grade annoying.
My comments are based on the assumption of 'conversational touch' which is what I think you mean rather than anything more overt.
An open question to the thread on the theme of touching, do women not find the hand in the small of the back thing rather condescending?