My understanding is that it really doesn't matter if the rear wheel locks up, as long as you remember to not release it -- then you risk high siding.
That is what they used to teach at the MSF courses. They would have you lock up the rear tire and come to a stop without releasing the rear brake. They don't do that any more. Like a lot of the stuff the MSF taught, it really wasn't true. If you lock up the rear brake and release it while still going straight, it's not a problem. If you're rear has started to slide out to the side and you release it you may high side. If your rear tire has started to slide out to the side and you do nothing you will likely low side.
The best way to understand braking is to spend some time riding in the dirt and practice sliding your rear tire. That and reading up on the dynamics of braking. There is NO Substitute for understanding how braking works and practicing. ABS will save you some of the time but it is also not a substitute for knowing how to ride your bike.
BTW, there are a lot of "facts" about braking that are floating around out there that people just accept as true since they have heard it so many times. Many of these "facts" are not true.
Some common myths:
-Don't use the front brake hard you will go over the handlebars.
-Don't brake in a curve, you will run wide and off the road.
-Braking is 70% front brake and 30%rear.
-I had to lay her down to avoid the crash
Also, ABS is not magic, it won't create traction where there isn't any. ABS is most effective if used by someone who understands what it will and will not do.
