We are required to do it annually, in SC. We have been fortunate, though, that we have never had problems with the shots in any of our pets. My cats go outside, so I do it for their protection, plus the fact that Buttercup has the tendency to be quite playful (that's a nice word for "bitey") and I feel better with him having his shots up to date at all times.

I would allow your vet to help you make that call. Seems as if they are inside, all the time, you shouldn't need to worry as much.

Good luck!
Logan