Just replaced my rear tires a few weeks ago. Got a nasty nail in the right rear that couldn't be fixed. Replaced both rears because I've never liked the idea of only replacing one tire (different circumference, newer fresh rubber, etc) but also because the old rear tires were almost down to the wear bars and needed to be replaced soon anyway. I have maybe 35-40% tread life left on the front and will get all 4 new tires when those wear out (I'm assuming the rears will have significant wear then too).
If you do get only rear tires, I'd at least advise you to get the same brand/model of tire that's on the front. Don't mix and match different tires, even if they are in the same performance class.