if you are a maniac, or live in a hot place, higher is probably better. the only thing it will do, is show up leaks that were borderline already. nissan use a 0.9 on NA motors, and a 1.2 on boosted ones, i use 1.2s because i have a similar effect on engines as a turbo ;-)
so far as boiling over is concerned, it isnt so much about that, thats what the overflow bottle is for. its more about controling hot spots inside the head.