Well the problem with what you've said is that the rear tire loses it's gyroscopic power as it's slowed down, the faster the wheels are moving the more upright the bike wants to stay, this is why you have to counter-steer. Now, just to "correct" your first sentence "You're only going to high-sided if you've allowed the rear wheel to step out from behind the bike though." 2 things need to happen, 1 the tire needs to slip out... but 2 you must RESUME tire movement before the tire is back in-line. So, locking the rear wheel isn't necessarily bad, but you should NOT let go of the brake until you are stopped or 100% sure the tire is inline with your path of movement. Contrary to a front wheel lock, where you should immediately release pressure and reapply. But either way, this is all moot, if you're smart enough to know all this AND apply it in a panic situation you're golden :-D