I doubt there are many generally accepted rules, but here is my take on it:

Two things to keep in mind:
1- you can get a whole lot more braking from the front than the back. When you brake, your weight shifts forward, pushes the front wheel into the ground, so you can brake harder before losing traction. In the back, the opposite happens... the rear un-weights, and is loses traction easier. So in ideal circumstances, you usually go harder on the front if you want to stop fast. This is why you often see larger rotors (more powerful) in the front than in the rear on mountain bikes.

However....

2- The consequences of losing traction in the front is usually a lot worse than losing it in the rear. So if you are in a situation where losing traction is highly likely (like coming down something steep and loose), go light (if at all) on the front, and be more bold with the rear. Accidentally locking the rear might kick the rear out to the side if you leave it locked too long, but locking the front can wipe you out very quickly.

One of the most important skills you develop as you ride more is to learn when to hit the brakes, and when to let off. For example, you may be in a rough section where you need to scrub some speed, but you don't want to be braking over the bigger bumps (that is a good way to go over the bars), and you also don't want to be front braking in the loose stuff. So what you end up doing is letting the brakes go over the bumps and loose stuff, and looking for places you can hit the front brake hard to scrub speed. Sometimes that is just a 2' patch of dirt, or a loose section that is at least straight so your front end does not slide out to one side as easily. I guess what I am getting at is that the better you get at knowing when you can brake and when not, the more you can use your front brake safely, and the more braking you will have available to you.

So, what I think is a good idea is to go a little heavier on the rear brake as you are learning how traction varies on different surfaces and features, but try to work towards having the front as where you do the majority of braking (when there is traction..... otherwise, you are sometimes just stuck with using the rear).

Regarding the advice you got about going down a steep hill: it really depends on the traction. If it is very loose, sure, go easy on the front and favor the rear.... fact is, there is not going to be a lot of braking force available to you no matter what you do. But if the surface is relatively clean and smooth (or if you find even a small section of it that is) go harder with the front as well.

The advice a previous poster gave regarding the need to shift your weight back when braking is spot on.