Actually, Ive found that it is probably one of the single most ticklish spots girls can have. Most of the time when you are playing around down there, she is aroused so you aren't likely to do much (nor do you really want to), but if something happens and you decide to get playful, you can usually change the atmosphere enough to relax her and make her more vulnerable to tickles.
But lemme warn you, that area is about 10000x more sensitive than the bottoms of feet. Keep out of arm flailing distance if you go for it =D And her laugh will prolly wake up neighbors.