When you're actually dating, that's fine. It happens.
But unless an ex ceases to be an ex and becomes a current again, she ought not to have any control over you, however minor. It's only a good idea to let anyone have any control over you if you receive some sort of benefit in return. Exes of any sort (ex-girlfriends, ex-bosses, ex-teachers, etc) are by definition no longer giving you benefit of any sort, so they ought not to have any control. And if they are giving you some sort of benefit (friends with benefits, for example), then they're not really an ex. They're just someone who's figured out how to get what they want without without letting you have any control over them.