Some schools allow it, some don't. You should know that some schools won't let you defer until you've enrolled, and even then, it's a case-by-case basis. The intent is usually some unforseen circumstance that prevents you from enrolling in the year you applied.
It is NOT so you can have a safety net as you apply to schools next year using the deferred enrollment as a back up. So it's a calculated risk - do you want to give up the admit you got this year to give it a go next year? Because I would expect re-applying to a school you were accepted at last year is pretty hard. Especially if your reason for not enrolling the previous year was "I think I can do better."