It varies, and often depends on the people and dojo involved. Most of the aikikai aikido dojo I visit respond to any questions by asking me to wear my belt and hakama.
When I go to train in Daito ryu, I wear a white belt, no hakama. No one ever looks at me funny for doing it, but most of them know I am ranked in aikido. So they don't mind throwing me hard
If people come to a yoshinkan dojo, and are not yudansha, but typically wear hakama in their own practice, I advise them to just wear their normal belt and dogi, no hakama. We typically don't wear the hakama as much, and it might seem a little strange. At the Inoue Dojo-cho seminar recently, Inoue Sensei was the only person in the room I remember with a hakama on. That included the 7th dan instructor who's dojo it was. Basically when in Rome...