You can soak the first stage as long it is under pressure (connected to a tank); the inside of the first stage is not supposed to get wet and the safest way to make sure water stays out is to pressurize it.
That's what I thought. Seemed a little like overkill to categorically say "don't soak the first stage." If I can spend an hour underwater with my reg attached to a tank, I'm sure it can stand an hour or two attached to a tank in a tub on my porch.
Tim