I don't know; it's mostly pretty natural. It depends a lot on where you're going next, too. For instance if you're about to jump up in the left hand, you might want to keep the thumb free, though it's not completely necessary, either. I just find that's a preference of mine. Usually I just use what fits easiest under my hand.
For instance, I had a chord in the right hand that was Bb, C#, F#, and Bb (ascending). My piano teacher asked if I was using 1-2-3-5, and I said no, 1-2-4-5 felt more natural and fit better under my hand. So I think it depends a lot on your hand.