Balance should be no problem at all. The oly bars have clearly marked rings and knurling so you can position your hands equally spaced from the center. You should be fine. It's all I've ever used.
There are a couple of threads going on at the moment on overhead pressing and front squatting for any other info on the topic.
Edit:
There is a big downside to using the EZ bar which is that it determines where you must place your hands rather than you being able to choose based on your size.
I don't even use the thing for curls in addition to which, for curls, it moves emphasis away from the biceps onto the brachialis.