The light and the dress both have influence on the white balance, because light reflects off of the dress. In practice, most of the light usually comes directly from the light source, walls, or reflectors you have. However, sometimes you will need to compensate for the color of the subject as well.

Scenario #1: You are taking pictures of a model in an ordinary studio. The model switches from a black dress to a white dress. You keep settings the same.

Scenario #2: You have been shooting pictures for an hour. The color from your lights is slightly different now. You need to adjust your white balance. (For some lighting, especially less-expensive lighting, the color will drift during a shoot. This applies especially to continuous lighting, but strobes also drift.)

Scenario #3: You are shooting in a room with white walls. You move to a room with yellow walls, and bring your lights with you. Even though your light is the same color, so much stray light is bouncing off the yellow walls you need to adjust the white balance.

Scenario #4: You are shooting close-ups of a model. The model changes from a black dress to a bright red dress. Enough red light is reflecting off the dress onto her face that it makes her face look unnaturally red. You adjust the white balance to compensate, or adjust the color of her face in post.

There are no solid "rules" for when or how you should adjust your white balance. A gray card isn't a replacement for trained eyes and artistic sense. Do use your eyes, and if it's important, look at shots on a calibrated monitor.