... or just change the water every day if soaking at room temperature and you won't get any nasties growing in the water. Leaving a brush soaking for a week in the counter without changing the water is asking for trouble.Id agree that the reason to use cold water and soaking it in the fridge is to prevent the growth of mold or bacteria in your new brush.
Leaving it soaking for a week in room temperature water is just asking for bacteria to grow.