Stanford doesn't even have those "registrar-imposed limits." Classes almost never fill up; they just get larger rooms or add TAs. Students don't usually register for classes until after the term has started!

I was an undergrad at a more traditional school where we registered for classes months in advance. I was amazed that the Stanford system was so informal, but in practice it worked just fine. It's wonderful to be able to audit a few classes before deciding on exactly what you're going to take; you may learn that you can't stand a professor, or love a surprising subject.