if they're just commuter hacks, they should be fine living outside if you keep them covered, washed and lubed etc. you'll get a bit of surface rust but that's here nor there. if you're riding in winter, make sure you rinse the salt off regularly (i had a pump-up plant sprayer for that, as i couldn't get a hose to reach).
for fancier bikes... inside only - you can get various storage hooks/racks to tidy them up a bit. compromise.. get the missus to ebay some shoes & handbags to free up some space
if you go down the shed/locker route, have a look here: http://www.bikeradar.com/road/feature/a ... rt-1-21131
"No se deja de pedalear cuando se envejece.
Se envejece cuando se deja de pedalear"