Since there are only 6 reformers in the studio it is best to sign up in advance to ensure you get a space in a class.  For AM classes (anything before noon), there is an 12hr cancellation window in order to avoid being charged for the class.  For PM classes (noon or later), there is a 6hr cancellation window.  If you aren't sure you can wake up for that 6am class, you may want to wait until the morning to register and can sign up as late as 15 minutes before class!  The risk is that you can't be guranteed a spot.