Your saying you "Shouldn't" timbre match your front 3 speakers?
What I'm saying is the we should be careful about terms and admit that timbre really is the character and ugly footprint of the speaker. Really good neutral and usually expensive speakers don't have a character, and then the issue becomes one of radiation angle or coverage to minimize interference of the three front speakers.
Now a lot of people will say that the speakers have to be made out of the same cone material, and even use the same driver compliment. I know this not to be true. For my left front and right speakers I selected drivers with magnesium alloy cones. For the center I chose drivers with polypropylene cones.
However the sound stage is seamless, and the speakers all have a neutral flat frequency response. It matters not whether a singer moves stage left right or center the sound of the voice is unchanged.
I stand by my comment that as speakers improve and come to sound more and more correct these aberrations we gloss over by by referring to as timbre will disappear. As I said you don't timbre match an amp do you? Of course not! This just shows that most speakers are falling far short of the performance of amplifiers.
All I would say is that if speakers are to have an aberration they should all have the same aberrations rather than different ones. It all boils down to intellectual honesty.