You want them to have as little tension as possible. Just enough to pull the pads off the wheel when you don't have the lever actuated. When you are trying to set your v-brake arm tension always try to loosen the side opposite the one touching first rather than tightening the side that is.
Tighter arm tension does not give you any better braking what so ever, it just makes actuation of the brakes stiffer, which isn't a good thing.
So to answer your question, there is not default rule of thumb setting. It depends on your levers, cables, etc...