In an ideal sense, truly "maximum" braking would occur with the rear tire right at the point of liftoff - with a downforce of zero pounds. Ideally, the rear brake would contribute 0% of the force under maximum braking.
So under that idealized situation, you wouldn't need to apply the rear brake because it would just lock up anyway. On the other hand, it wouldn't matter if you did, because zero lbs of downforce isn't adding any lateral stability either.
Someone "crunched the numbers" once and found that a "single" road bike on flat ground can decelerate at a maximum of about 0.6g before the rear tire will lift. It is important to note that most cars can do better than that... and that you'll probably be less than 0.6g if you're going downhill, which is probably why you're braking in the first place.
If you're riding on some super-sketchy surface, front-brake-only is obviously not the smartest move.
Is a little rear brake here or there going to hurt anything? Probably not. Just remember that your front brake is your primary stopper in most cases and you'll be fine.