This is something I've been thinking about a lot lately. Lobing is a real issue with horizontal center channel speakers, but it's a compromise we make to get the speaker to fit under or over the screen while maintaining driver number and cabinet size.
So people recommend identical speakers across the front when possible. This is generally not possible when using large tower speakers, even in a dedicated HT.
Does anyone use a smaller bookshelf as a center? For example, putting an Axiom M22 as a center to complement M60 tower mains.