It all depends on terrain, and speed. From what I've noticed if you wanna stop really fast, slam on the rear first all the way, then half-3/4th the front brake. Actually sliding the bike sideways stops you pretty quick too.
In a staight line, I tend to apply the front a little (1/4), then the rear all the way(100%), then the front some more until I stop (1/2-100%)... I also shift my weight as far back on the bike as possible without letting go of the brakes or clutch. So in that regard I'd say rear 60% front 40% but sometimes it'll go rear 80 front 20, or even front 90 rear 10... it all depends on terrain.
Grass: 40:60 (front to rear)
Sand: 40:60-50:50 depending on pack
Gravel: 40:60-50:50
Tar: 20:80
Mud: 50:50-30:70 depending on depth/speed
In some cases if you use them both equally then you're defeating the purpose of a back brake considering you'll just endo regarless. I think this is why the general rule is to tap the rear first as to avoid flying over the bars.
Again, it all depends on your style, the terrain, your speed, and your ability.
Maybe someone else will have a little more insight.
Dan