I think this is something that's partially true.

Yes, generally speaking the leader gets to choose which figures will be danced and the follower doesn't which means the follower may not get to dance all the steps she knows. However, the leader shouldn't necessarily dance all the steps he knows in every single dance either. Especially with someone you've never danced with before, it's best to start with the basics and simple figures and work up until you reach the follower's limit (she starts having trouble following figures). Then you know about the maximum level you can dance at with that follower. This level may not be equal to your maximum level, so you're not necessarily guaranteed to dance all the figures you know as a leader either. If someone can follow high level figures and you can lead them, feel free to dance them. But you shouldn't assume the lady can follow everything you can lead.
What you are kind of guaranteed as a leader is that you will only dance figures you do know. A follower has the opportunity to dance with many different leaders, each with his own repertoire of figures and patterns and with certain specific (maybe favorite) figures he's best at. Over the course of a night, the follower may get to dance every figure she knows by dancing with many different men. If she mixes around a lot, she will probably get a chance to dance lots of moves, maybe even some she hasn't learned in classes. So while the leader is limited to dancing only what he knows every time he gets on the floor, the follower has the advantage of never knowing *exactly* what she's going to get from her partner and can end up dancing a much greater variety of figures.
That being said, I don't think it's necessary to ask which part you want to dance before every single song. Not everyone knows the other part well enough to dance it. Some people may be quite happy dancing only their part all the time. However, it is good to learn the other part because it gives you a much better feeling of what your partner has to go through. Leaders trying to follow will learn how good and bad leads feel and how to wait for them without anticipating or backleading. Followers trying to lead will find out how much the men have to think about in general while trying to still keep the dance enjoyable.

You will certainly gain some useful knowledge and experience by learning the opposite part if you have the time to do so.
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James Marshall
marshall@astro.umd.edu http://www.astro.umd.edu/~marshall