Do you mix single volumes and omnibuses?

I was just talking with a friend of mine whose got a small collection himself and he showed me, and he had a good number of Fire Force with mixed singles and omnibuses. He asked if it was odd but I couldn't really give him an answer. It's not what I do, and seeing everybody's collection here, I don't ever remember seeing somebody else do it either.

I usually maintain which format because sometimes the Omnibuses are printed larger, or there's extra art between the volumes like in the Inuyasha's.