I stick with just the rabies for indoor only cats, as well.

If you are really concerned, the vet can do a titer test, to see what level the immunity is at. That will cost as well, but lets you know if the cat has any protection.

Keep in mind that the "every 2 years" requirements are often set by state law, NOT by the need for the vaccine. For years, up until this past March, here in RI, our rabies shots had to be updated every 2 years. Massachusetts and 37 other states give the SAME vaccine, the SAME dose and manufacturer, and it was good for 3 years in those states. It took a HUGE push for us to get the legislation passed, to finally have it recognized as valid for 3 years here.

As Randi mentioned, if you board your cats when you travel, you may need a different response.

Also if you are doing any type of rescue work and regularly bring strays into your home, you should consider being more proactive on the vaccines for your own cats. Most rescue groups will advise you to keep your own pets safe, as a priority, and which vaccines to maintain.