Right, so first, I'm an American living in the UK (this should answer the previous poster who inquired about the number of Americans on this board.)
When i sent out wedding invites, I addressed them very formally, using conventions provided Debretts. So many rules! But, basically if the invite was being sent to a married couple, I addressed the inside invitation to "Mr and Mrs John Doe". I received complaints, including from my FMIL, about being referred to as Mrs + Husband's Name, or Mrs generally. Honestly, I don't like Mrs -- don't need to get into the details of why -- but I prefer Ms, whether married or unmarried, and the vast majority of my girlfriends feel the same. But, to each her own, and I don't care whether you personally use one or the other.
Now, when I recently sent out invitations to the dinner the night before our wedding (these invites were very informal), I uniformly addressed all females as Ms, thinking I would eliminate the Mrs complaints. Now I have received complaints, including from my FSIL, about Ms! (And it didn't come off as a "ha ha, lol, did you forget I'm married" type of complaint.) I mentioned this to my H2B, and he said that women in the UK conventionally use Mrs if they're married, and are offended if you call them Ms. However, my understanding (at least from Wikipedia, which knows all things...hehe) is that "Ms" is the default regardless of marital status.
In the interest of full disclosure, the FSIL's complaint could have stemmed from that I'm not changing my last name upon marriage, formally or informally? I completely support those that want to, but I don't for a variety of professional and personal reasons.