I think either way is fine. I varied mine based on who I was sending it to - older relatives and my parents friends got the more formal "Mr. and Mrs.," while for my friends, I tended to just do "John and Jane Smith." I will freely admit there is no basis in etiquette for how I handled it, but since save-the-dates are less formal, I figured I would go with what I knew the recipient would be more comfortable with. For my formal invitations, I'm going to go with formal address regardless of the recipient.
I just addressed them with their first names, no Mr. or Mrs (Sue and Joe Smith). The invitations will be more formal. I think whatever you want to do will be fine. STDs are less formal than invitations.
Re: STD Question
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