I voted for trivia purposes, but really it doesn't matter much at all. The real value is in being able to climax completely without having to break of contact. What happens to the jizz after that is of little concern.
If really hates to swallow, I'd have no interest in bribing her to do something very far outside her comfort zone. On the other hand, if she thinks nothing of swallowing, why would I pay her anything extra?
However if she did something extra entertaining on her own - like letting it run out slowly, blowing bubbles or gargling, I might throw in a tip for appreciation.