I wouldn't.
I'm probably weaker then most girls anyway, and I absolutely hate physical violence. If a girl were to hit me I would not hit back, nor would I if a guy hit me. It doesn't matter to me, I cant stand to be very physically violent, and haven't been in a while.
Whether it's right... that's all very circumstantial.
I'll immediately say that I disagree with all of those people who say you probably deserve it if they do hit you, as that just isn't true. I have been pinned into a corner by a group of both genders, and they seemed to have the same reason for it; they were bullies. So that's not true.
I guess it could be considered fair in some ways, but I don't agree with retribution, so I'll say don't hit back unless she is attacking you, rather then just striking one hit. If it's attacking, then it's appropriate to hit back, but otherwise I cant justify it.