I am finding that a few of my clients are choosing to not address their inner envelope - basically, because they are lazy. (yes, I am talking about you) This is a VERY important aspect of your wedding. This tedious task allows you to tell your friends, or family member who exactly you are inviting. If you are only inviting Mr. Steven Smith, and not his fling of the month, the inner envelope would only say Mr. Steven Smith, not Mr. Steven Smith & Guest. Or if you are not inviting children to the wedding - it would be Mr. and Mrs. Martin Johns - not The Johns' Family.
It is a definite pet peeve of mine to see that I sent out invitations and then when they are sent back someone erroneously added a +1 to the response card. This small task can eliminate a lot of heart ache. You haven't been lazy on every othe detail of your wedding, why skimp here?