I think it's always worth it to tell your friends how you feel. The longer you don't call her on it, the harder it is likely to become. I would want to get it out in the open, so that she know that it hurts when she uses the wrong pronouns, and that you value her as a friend nonetheless. But I know how hard it can be to find the right time to do that.
When I am dealing with friends, what I have found works is to mention it pretty much every time they do it, as soon as is socially convenient. With those few people who are really bad with pronouns, I don't belabor the issue; I mention it once or twice when they first screw up, and then drop it until the next time I see them.