Ah but see Lev, a handshake or a hug are all actions that the other person acknowledges and agrees to before the contact is made. You extend your hand, I accept it and we shake hands. You open your arms, I open mine and accept the hug. So you see in those situations the permission is there, even if not verbal.
With Barbers, Doctors etc. I know going in that they need to touch me and again my agreement to the contact is implied by my very presence. The touch is absolutely necessary in order for them to do their jobs. As a photographer though the same is not true. I can make a photograph without you having any reasonable expectation that I will need to touch you. That is not to say that you won't need to touch me but unless I know it going in I would not anticipate that it would be required.
With a model for me again it is more about respecting the personal space then anything else. While I do make it known to the model in advance that I may need to touch her to communicate a pose or adjust her hair or wardrobe, I still feel that for me it is appropriate to simply ask permission before touching her. For me it is a show of respect.
Like you a lot of the girls I work with while of legal age are still quite young and I don't want them to leave my studio misunderstanding anything that occurred while they were there. Unlike you I often work with models who are absolute strangers to me. Oftentimes I have only seen a few photos of her and exchanged a few emails to arrange the shoot. In most cases I do not actually meet the model until she walks through the doors of the studio so the trust factor is not already inherently in place. I have to build it from the moment I meet my model.
Don't get me wrong, I m not saying that the way you work with your models is wrong, as you have already said, you already know your model by the time she gets to the studio. For most of us though that is not the case. I as a general rule only book my shoot calendar 30 days in advance because I don't like to fill it up with TFP shoots and then have to cancel them because a paid shoot came along. Even at 30 days in advance I sometimes will have to cancel in favor of a paid assignment so I keep the scheduling as short as possible.