Each rider has their own personal cockpit setup that they feel comfortable with. Position of shifters and brake levers can vary depending on the rider. For some riders this is a very meticulous affair and others may have not given it much thought. Bicycling has published a nice article on mountain bike brake lever adjustment and position. The article details how the brake levers should fit your fingers and adjustments you can make to your brake setup to suit your riding style.
I’m glad they covered one of the best tips I knew of for those of us that tend to ride steep terrain.
“if you frequently tackle steep and demanding downhills, angle the levers up slightly so that your fingers, hands, and arms are in a straight line while you’re in your downhill position on the bike.” – Bicycling.com