Our back up is in the garage. It gets used easily twice a month or so, so I can check if there's any damage to it. When the babysitter picked DS up from preschool, it got used once a week. I just got the Safeguard GO the other week, need to play with it yet. It folds up into a bag, so that can go in DS's closet, if I ever clean it out! Our garage is dry, it's just a little dusty. I have a car seat travel bag that I store our spare seat in.
I will say that having a spare seat was a brillant idea of DH's. I never have to uninstall our seats if DS rides with someone else. I had a backup to use when DS threwup all over the car seat. I've had to pick a friend's DS up from school in an emergency and we didn't have to work out car seats. We travel a lot and our spare seat weighs considerably less than the other seats, so it's very easy to travel with. I would never take our Safeguard or Como on a plane....those two are very heavy seats..I can barely carry the Safeguard to the car. If the Go works out for us, travel will be even easier.