If you can get it off the front I would definitely do that, especially if its wireless and you don't have to deal with ugly (and prone to fail) wires...
Much of the time the sensors go on front simply to lessen the wire run or distance for the wireless transmission.
I have a Polar S720 and it previously didn't have the range to mount the speed sensor on the rear, but I found some info (on this site
) about changing a jumper to increase the transmission strength of the sensor for a slight loss in battery life (worth it)
Perhaps the Cateye sensors have the same functionality if it turns out the wireless distance makes the sensor spotty...
Oh, and if you can spend a bit on a bike computer/HRM I recommend the Polar S720... I used the HAC4 from Ciclosport for a while and just found the Polar a better, easier to use, more complete unit, though they are both excellent...
Good Luck!
Feanor