Its never too late to register. It really doesn't matter if you register. They just recommend that you do so it will save you time when your in labor.
You can pre register at any point up until you're in labor (at which time you'd no longer be PRE registering). It makes it more convenient if you show up in active labor ore transition so you don't have to fill out paperwork between contractions.
I didn't register with either of my pregnancies. I'm not really sure that preregistering is really all that important.