Yeah, a lot of people at my school had that problem last year. It turned into a huge battle between the students, the guy who was running it and the parents, simply because the majority of people were not getting the subjects they wanted. It would have been much worse, but my school offers a lot of online courses, so people could do their subject there, but for those who didn't like that idea kinda got screwed.
If it does turn up, try speaking to the teacher ASAP, and try to get him to be sympethetic. That way you might get the advantage when he reshuffles them. I know everytime they moved subjects to make things fit everybody just groaned because it was worse than before.
You should be alright if the problem is that their is one line where you have nothing you want to do, but if you have a few stacked up in a line, you may have a problem. Sorry if I'm scaring you even more ;-)
Good luck.
Edit: I do 2 extensions, it ain't that bad. I'd advise starting the year with 14 units, and choosing to drop on line fairly quickly. That way you have a free line, and have the same amount of unit as everyone else. Just make sure you alright to continue with both extensions, otherwise your stuffed because you can't drop one.