Our dojo's sensei passed away a few years ago, and since then, his senior students teach all of the classes. We do not refer to any of the current instructors in our dojo as sensei, but rather address them by their first names. This is done more out of respect for our late Sensei, and to preserve our memories of him. The only people we call sensei in our dojo nowadays are guest instructors from either Aikikai Hombu dojo or outside our association. I guess it's just each to his/her own...
We do the same at our dojo whenever Sensei tells a student to lead the class.