I'm going to piggy back from what gigglemakr and tickleron mentioned before.
I like tickling a woman's armpits for a few reasons. There is that mystery as gigglemakr said about women not usually showing their underarms unless reaching up for something in a sleeveless top. That adds an intimacy and secrecy to it that goes hand in hand (hand under arm?) with the whole intimacy of tickling itself. I also like being close up to the 'lee's face as I tickle her armpits; it was one of the charms about the true story I posted about Serena a while back. Finally, I like the feel of a woman's armpit. While she gets tickled, there'll be the warmth and the moisture, but there's also the variety of either smooth or stubble, depending on what her morning routine was.
Anyways, count this one as a wordy endorsement for tickling a woman's underarms! 🙂