Do you keep 'em after installing new ones? I wasn't sure if there was any need to hold onto "backups" or not.

Its always a good policy to keep the good old parts for a while after you replace something.....just in case?
When I replaced my belts (accessory and super charger) I put them in the spare tire well ...just in case

I've never had a spark plug or ignition wire fail me after I replaced them. But I do keep the old serpentine belt in the spare tire well too.

I've had a plug wire fail on the road. Definitely keep at least the 2 longest wires. They'll work in any location in a pinch. Toss the plugs.

hmm i might wanna get me a serp belt in the spare tire do but doesnt it get old sitting there or weaker?

I hear what you're saying. Yeah, an old serpentine belt may degrade a little bit just sitting around. But no where near to the point of falling apart, even sitting there a long time. Remember, just like your spare tire, you would be using it(if you had to) for a short time until you replaced the belt with a new one.