Do I have to shake her hand when done or will a knuckle tap and good job do?
Just pat her on the head.
I too shut off the taps and open the bathroom door with the towel. I, however, have no problem shaking hands after a match. But if I was grossed out by someone I would go wash my hands and if I knew for a fact they didn't wash their hands after going to the bathroom I would probably tell them I will not shake their hand for that reason. Maybe it will sink in to wash up.
I don't think I have OCD but I will not eat or touch my mouth or eyes after shaking someones hand until I get a chance to wash up.