To me, it really depends on the type of personalities of the ticklee, and the other people around.
In college, I was close friends with a girl, whose boyfriend, who also happened to have been a close friend, had a bad temper. He would easily go off on someone who tickled his girlfriend for no reason, and yet, on one or two occasions, he tickled his girlfriend, and I (as well as several other friends who were present) helped him, and he was cool with the whole thing.
Sometimes, if you have people who are laid back about such things, it's not that difficult. The problem here is that so many mainstream girls I've come across hate to be tickled, so, in that case, even a playful tickle could easily get someone mad.
My best advice, take stock of the person you are going tickle, their personality as you know it, the situation, and who else is around, and make the judgement call from there. With friends, people can generally get a feeling of what would be okay, and what wouldnt. Somewhere along the line, under the right circumstances, I'm sure you'll be able to get some tickles in. Hope that helped.
Mitch