Dumb question re: addresing invites (not wedding)

I'm writing thank-you notes for my shower and am not sure how to address the envelopes.
For single girls I'm doing:
Ms. Sarah Smith

If they are married can I use Mrs. Sarah Smith or is that wrong?
I *hate* Mrs. Joe Smith..I want to use the girls' names

Single is Ms. Sarah Smith

If they are married, send it to Mrs. Sarah Smith

thanks