In a perfect world, we would all run five or seven identical speakers for all floor channels, heck even suspending some for overhead channels. However, the world isn't perfect, and I'll go against the grain here and say that I don't think timbre matching is very important for a center channel. I know that's the prevailing wisdom, but my experience has taught me different.
To me, the absolute best setups I've owned and heard are where all speakers are of very high quality. The differences between these speakers, even one brand or design criteria to the next is significantly smaller than the audible differences between the bottom shelf, big box store offerings. Add to that that most HT setups run some form of advanced room correction and the differences get even smaller. For instance (because lets face it, we're talking mostly about tweeters here), I have mixed diamond (802 D2) and ribbon (ML Focus) tweeters, and currently mix beryllium (Focal Sopra 2) and ribbon tweeters. Both setups sound far superior to all ribbon (ML 60XT + Focus), all aluminum (B&W CM9 S2 + CMC2 S2), or other mixings thereof where the speakers were not as high quality, especially after Anthem room correction.
Your center speaker IS your main speaker in a HT setup, and as such will be almost single handedly handling all of the dialogue and on screen effects. The mains and surrounds are there mainly to fill out orchestral scores and off screen effects. You system though will sound pretty much like your center channel and subs.
To me, the absolute best setups I've owned and heard are where all speakers are of very high quality. The differences between these speakers, even one brand or design criteria to the next is significantly smaller than the audible differences between the bottom shelf, big box store offerings. Add to that that most HT setups run some form of advanced room correction and the differences get even smaller. For instance (because lets face it, we're talking mostly about tweeters here), I have mixed diamond (802 D2) and ribbon (ML Focus) tweeters, and currently mix beryllium (Focal Sopra 2) and ribbon tweeters. Both setups sound far superior to all ribbon (ML 60XT + Focus), all aluminum (B&W CM9 S2 + CMC2 S2), or other mixings thereof where the speakers were not as high quality, especially after Anthem room correction.
Your center speaker IS your main speaker in a HT setup, and as such will be almost single handedly handling all of the dialogue and on screen effects. The mains and surrounds are there mainly to fill out orchestral scores and off screen effects. You system though will sound pretty much like your center channel and subs.