I've been in a relationship for almost 6 months now, and at the beginning she couldn't STAND to be tickled. Every time I could, I'd dig into her ribs, or run a finger across a sock, and she'd scream out. After a while, we were talking about what "gets us going", and I mentioned that tickling her does, especially her feet. Since then, she's become more tolerant, and she actually enjoys it now, because she knows it gets me going. It's gotten to the point where she will put her bare feet on my lap and say "What are you going to do to me now?"
Just look for the right time where you can tell her. If she's open to it, you won't have any problem.
