In the dojo where I practice (Aikikai), we change partners only once during regular classes. However during seminars, we change after every technique. Our late Sensei was very adamant that the most senior students practice with the beginners, and we continue that tradition. On the other hand, sometimes some students will practice with each other more frequently than others. For example, my sempai and I will often try to seek each other out to practice with each other. This isn't so much considered preferential treatment as much as trying to foster and nourish the sempai/kohai traditions in our art and dojo. I hope that someday when the time comes, I will also be able to act as sempai for another student.