I use about 70% front brake and 30% rear most of the time. Using more front brake gives you much more control while descending. The only time I use ONLY my rear brake, is when I dropping down a section and want to change my line. I tap the rear brake and simultaneously shift my body weight in the direction I want to go. The rear brake pulse will shift the bike into the new line. Other than that, I can't really think of when you would use just the rear.

L8R