Do you require clients sign a contract for SEO services?

I'm offering month by month SEO with no obligation on the client to sign for x amount of time. Do you still recommend I set up a contract? I'm only concerned of the client claiming I promised X results by Y date, and it'll be my word against his. So I'd like to get it in writing that there's no guarantees. But is it necessary? It's another step a prospect must take before paying, and it seems like most agencies don't require one for SEO(?).