Back when I was younger, I used to feel that if I was woman enough to actually let a man orgasm in my mouth then I would be woman enough to swallow it.
Kindof was a right of passage back in the day!
Now, men seem thrilled when it happens.  
Women are moody creatures.  Sometimes, it's just a hot thing to do.  And giving.  Other times, maybe it's not something that I feel like doing and it's not as pleasant.
It's always different.  And the moment of time when you're with a man, and the chemistry that is going on (which is often very good!) matters a lot.
This is an honest answer.  Swallowing can be fun.  I like it.  But it's not ALWAYS the best activity.