Depends on how much too fast you ride ?.
Ride like me and the same amount of coverage I use on road is enough, possibly covering the brake if I hit something unexpected.
Riding in trees or similar I can certainly recommend it. You lose about a second and a half moving that finger ?. The point of riding with the brake covered is that you get a reflex 'clench' when 'oh shit!' happens, you'll have the front brake starting to work before you are aware of the hazard - by the time brain takes over and it can decide whether you need more brake or less you may have already saved yourself from a crash.