We're using one of those grayhill encoders (stole this one from our 2007 robot until we buy one)... here's the mount we made yesterday for it:
It's slightly spring loaded since the belt isn't exactly sticky. When the robot is turned off, it's very easy to rotate the turret, so before each match we simply turn it to whatever its initial position is going to be and start. We'll also have reset limit switches so if for some reason it randomly slips it'll still be able to "fix" itself.