I recently saw a wedding cake that had an unfortunate monogram on it. It proudly proclaimed "DOA" on it.
Always check to see what your monogram will spell before you elect to use it on your cake.
I've had clients who decided to just use the last initial of the groom's name when they realized that their monograms would spell something "interesting."
"EPT" is one that comes to mind. They decided that if they put that on their cake their mothers might think they were announcing a pregnancy at their reception, so they went with a single "P" instead. I've also had couples decide against using "SOL" and "CIA" in order to not be mocked by the groomsmen.
The correct form for an individual's monogram is first name, last name, middle name. Once you get married, the married couple's monogram would be the bride's first initial, the groom's last name, and the groom's first name. Remember "ladies first" and that's the order it should be in.
Of course, if a couple wants to do it in a different order, or use a different letter, so what? One couple I worked with wanted an initial, but the bride wasn't going to change her last name. Since both of their first names started with the same letter they decided to use that instead.
And now I will use my best friend as an example of what to watch out for. When I met her in college her initials were "ASS". So no matter how she changed the letters around, she was in trouble. She married a man whose last name also starts with an "S" so she's pretty much out of luck. Their married monogram would be "ASR" though, so she could use that if she so desires.
Or I could carry through on my threat of buying her towels with her personal monogram on them. That would be interesting when she has houseguests.