0 is the smallest size.
If you're out hunting and the spring breaks, you wouldn't notice it until you needed a rapid follow-up shot. You wouldn't have enough time to swap the bolthead.
I'd be willing to bet that if that ever happened you would've probably forgotten about putting a chunk of ice cold steel with a hook on it in your pocket anyway.
If you did remember and dropped one or the other into tall grass, mud, or snow, etc. you'd be out a heck of a lot more money than a rubber band would cost.
Spring breakage won't ruin the day unless you're shooting a timed event in competition, or trying to halt a swarm of charging fuzzie-wuzzies.
Seriously though, it's your money so if having a spare makes you feel better go ahead and get one. It can't hurt. Just be sure the bolthead you get fits your rifle as well as the one that's in it.