Hell no. This is YOUR wedding, so invite whomever you'd like to share the day. I agree with the 6 months or more thing. You don't want some ring-in sharing one of the most intimate days of your life.

We couldn't have fitted people in if they'd brought +1 - it was a expensive squeeze with the guests we could fit. Though we did have someone we fitted in at the last minute (ie, the day before), but that was our choice, not pressure from someone else.