I'm also considering having bra cups sewn into my dress. I did ask what the seamstress thought over the phone and she said she usually only recommends that for people with cup sizes B or smaller, so that you get the necessary support. Unfortunately, I'm a C cup, but I do have a corset top on my dress, so we'll see what she thinks at the appointment.
Things to bring to your alterations appointment:
-Undergarments you plan to wear on the day of
-Shoes you plan to wear (or those of a similar height)