I don't know of anyplace where you can safely get by with only a tetanus every year for your horses. Whether you keep them on your property all the time or not. But that may just be me.
My personal thought is that if that's all you intend to do, then your emergency vet fund better be a lot bigger than mine.
Here in Australia, at least my part, we only vaccinate for tetanus and strangles which comes in a 2 way shot. One needle, once a year.
I don't know where the OP is located, but if she were in Australia that is entirely plausible.