For one thing, you might end up with a situation where you write a story to a title and then discover it no longer fit.
For another, it's a distraction from finishing the story. Thinking about the title is not thinking about moving the story forward.
Still, when the prince is cantering home from his successful quest to learn how to break a spell, he's going to need a title for his story, I think.
Then I remind myself that he's going to be revised at least once, put through the writing group, and then revised again. So it's still a distraction.
OTOH, with the story in hand, there is no question of its not fitting except its being badly chosen now.