You really should rely mostly on the front. I rely on the front and only use the back stabilize the bike, or to be more clear, to keep the bike's weight from putting too much force on the front tire. But using it a lot and even more than the front really can cause the rear to lock. But the problem is you may not realize it till it's too late because when you use it more than the front all the time, you are training your natural reflexes to go to that in an emergency stop. And thats when it WILL lock the rear so easily you will be shocked. I use the front much more, yet even then I locked the rear once in an emergency. Those speedys have a really strong rear brake, which IMO is not a good thing. I think all bikes rears should be weak to the point if you step hard on it they won't lock. A drum would IMO ideal ! But on the front you want state of the art.
In any case, be careful ! I really think it's much safer to use the fronts mostly, if for no other reason than so that you don't train your brain to slam on the rear in an emergency stop. I couldn't believe how easy my speedy's rear would lock. The Bird's is too strong too, but much reduced over the speedys and i gotta think they did that on purpose because the fronts are stronger than the speedys.