A race is a race, so why not fight until the end? Now if its just a local race and you're throwing elbows or something like that, that's probably taking it too far regardless of gender. But why give up time and/or placings just to be "nice". Be nice on and off the course and race fiercely on it.
Now I did let a younger girl out kick me at the end of a 10k once, but the circumstances were atypical. I got to the race late and started over 3 minutes after the gun as a result. After working my way through most of the field (~500) I just didn't feel like kicking it for a 14th vs 15th place finish, aside from my legs being shot by that point. She did kick it in for all she was worth though. Incidentally I believe my on course time was either 8 seconds faster or slower than the winner, so I was a little pissed I got there so late.
Another time at a local road race, much to my surprise I had a woman on my heels towards the finish. It was for the win, so I out kicked her with no hesitation. Turned out she was a 2x Olympian.