Should the wording of the addresses on the save the date be as formal as wedding invitations? For example, should it read Mr. and Mrs. John Smith? Or should it be more casual, such as Mary and John Smith? The wedding is semi-formal and the save the date reflects that feel.
Also, if guests are included, should the address on the save the date reflect that as well? Such as, Mr. John Smith and guest? Or does the save the date just to go Mr. John Smith, and then the wedding invitation reflects the "and guest"?
I would not include guest at this moment only to him or her. It should be formal and reflect the nature of the wedding as well. The reason i say not to put a guest is what if you run into money issues or space requirements or you need to invite your cousin but Mr smith you have already invited a guest due to the STD. i would only send your STD to family, leave out coworkers and friends until you send out the invite because relationships can change very quickly
For formal weddings, use formal address. First, it's the first things guests see about your wedding and first impressions matter. Second, it'll give you a good feel for how long addressing will take with the invitations. Third, it gives the formal address a test run, so if there's a Ms. instead of a Miss or a Mr. that prefers Dr., you find out before the formal invitations go out.
Don't mention random "and guests" for the STDs unless you're 100% sure you can accommodate them financially and venue wise. If the SO is "permanent", you can mention them. If it's more of a dating relationship, no mention of the SO. If you've made a mistake and someone is suddenly more or less serious than you listed, the guest has the opportunity to set that straight before you need to address invitations.
Remember, "and guest" is only used if it's any random person in their life. If they have an SO, you need to call and get their name.