One theory on the matter...
I think you have to read the person very carefully and understand whether or not your relationship with them allows for this kind of touch. Some people try to be polite even when a person makes them uncomfortable, and they might not make their feelings clear verbally. Watch body language, and if you have the willpower (which I would hope), wait until they do something tactile to you first. If she tickles you, then I guess it's not a big deal to return the favor, provided it is brief and not intense. I'm a quiet person myself and I try not to offend people. Several times I've worked with or around people who were playful types, some of them of the creepy variety. When they would torment me, I had to pretend to think it was amusing because I was at work and there were people around. When you work with the public or even with certain types of employers, you have to put on a hospitable, professional attitude. Getting snappy and saying "Don't touch me" could not only tick off the boss and co-worker, it could egg on or make an enemy of the person doing the touching. If someone was consistently trying to touch me, I would start to wonder about them, and perhaps would feel safer telling the boss than I would confronting the person. I'm not implying this was the case here, but it might be one reason why people tell someone they trust ahead of the person they feel assaulted by.