I don't feel entitled to advice anyone about relationships, but I really hope what I have to say may be helpful.
I think in a case like you described, I believe would be wise to increase the tickling little by little and observe her reactions, and if she tickles back, and if she is actually trying to stop you
Personally, I've had sexual relations with two women who were ticklephiles, and a serious relationship with one of them. So it was my girlfriend who instigated the tickling at the beginning, and I gave clues and openess. We actually never actually made an verbal agreement. We had tickle fights, lots of verbal teasing, and soon we were tickling each tied up. If you date a ticklephile for a while, you will know it. Period.
Now, I had a long relationship with a woman who liked tickling in a flirty, but not sexual way. In this case, I kinda of went along the ride observing her reactions. If she tickled me, I'd tickle her back. It's curious, because she could bare to be held down and tickled for a while, and she had a considerable bdsm fetish, but she would never enjoy to be tickled while restrained. Curious, right? (I think women has more "shades of grey", because I have female friends who admtly enjoy tickling, but it seems in a more natural way) We didn't do "tickle porn stuff", and it was cool anyway.
So, the thing is, tickling may not be a "popular" fetish, but it's not pee or fart or whatever. People who don't have the fetish may enjoy it in certain levels, specially with loved ones. Light sensual tickling can be very pleasant. Don't consider only the scenario with stocks and safeword or whatever.