I can't see spokes breaking for being too loose. Just the other way round; I can see spokes breaking for being overtightened and thus being subject to a force of traction beyond its breaking point.
say you have a perfectly trued wheel. Say you loosen each nipple by one turn. the sum of forces still equals to 0, the hub is perfectly centered, but every spoke has less tension than before.
Now say that you put the wheel under extreme acceleration, weight or braking. In any case, the force tends to stretch some spokes, and to bend some others. The latter will never break. The former should have less chances to break now as the nipples are one turn on the loose side.
Let's get to the extreme. Say that we tighten the spokes to the very limit, to the point that each one is in the verge of breaking. The wheel will be perfectly centered, but any force that breaks the balance of forces -such as braking- will cause at least one spoke to break.
I don't get paranoid about spoke tension. When I lace my wheels, once they are trued I go one by one and give a little tightening to any loose spokes, and call it a day. Spokes will stretch with use and will loosen but the only thing that could happen because of that is that your wheel gets out of true, not that the spokes will have more chances to break.
At least that's the way I see it.