I say wear them till the rears are shot. I find I get more tire wear in the rear so you will chew thru those first I bet. If you are concerned about gaining the nicer ride of non-RFT then changing just the rears won't help. You'll still get all the hard bumps thru the steering wheel.
Once you need new rears...go for 4 non- RFT. I have a set of michelin pilot super sports and they are great.