A bride recently asked me if she orders thank you notecards to send notes for engagement gifts is it okay to just put the couple's first names, like: Jack & Jill or should they use the monogram they are creating for their wedding.
My answer: You can put whatever you want on your thank you cards but it's not appropriate to use your married initial until after you're married. Many stationers will suggest that you create a monogram using your first initials for the wedding invitation because you aren't married and using your new last name until after the wedding.
So technically you can create a married monogram for items at the cocktail hour (if you have your ceremony first) and the wedding reception. But stick to your first names and your maiden name until you say, "I do."