I personally think they don't have a preference. I always seen Viktor and Yuuri as a couple who switches alot. Topping, and bottoming would be like an emotional experience for both of them for different reasons. I think it's all have to do with atmosphere, emotions and situation at the time they do it. But I don't believe those two have hot sex very often. Instead they make love. They would frequently have highly emotional, spiritual and intimate love making. Even a simple oral would be intense.
this is an Extremely Good headcanon and probably canon and i love it!! ♥