I've always used SMB's and I've always done them front only, I guess because I feel there is more pressure on the forehand generally and so more of a need for protection on the forelegs. It has worked out fine for me this way so far.
Abo****ely, there is no reason you couldn't do so. If you feel you need protection in the front tho, chances are whatever it is you are doing would be more ideal if you also had some leg wear in the back. What color did you pick?