Definitely brake on when cold-starting a saw. When warm-starting it, I don't do it (probably should).
While cutting: As long as I'm holding the saw solidly with two hands, and not moving ***much***, I leave the brake off. This means if I'm limbing by walking along the trunk, I leave it off, and hit each branch. If I have to take a hand off the saw (for example to pick up a wedge and put it back in my pocket, or to steady myself as I climb around), the brake goes on first. Also, since we are in pretty steep terrain, I put the brake on even when just moving a few steps, if it looks like I might slip (which is quite common, I've had to throw the saw into the dirt many times).
When moving more than a few steps, I actually turn the saw off, and take the earmuffs down. I just don't like wandering around in potentially hostile territory (steep slopes, slippery places, occasionaly rattlesnakes) with my senses blocked out by the earmuffs. That's just me, I understand that other people can work all day in the forest with earmuffs on and an ipod underneath it, it just spooks me. Fortunately, all my saws restart on a single quick pull when warm (if they don't, they get readjusted until they do again).