It just depends on whether any of your kitties are "daredevils". In the 20 years that we've lived here, Pepper is the only one who has jumped from our second story deck. The other kitties are just happy to sun themselves and watch the birds.

Definitely keep them from walking the railing/banister. It would be easy to slip of of that if they feel the need to get up there and practice their balance beam routine.

Also, we've installed a sliding screen door so that we can enjoy the fresh air knowing that Pepper can't get out there. Halo and Cammie will sit out there for hours quite content, but Pepper can't resist the temptation.