I don't where you have been reading, but over the last two years I've posted to many threads about center speakers.
Now there have been many posts to this thread I need to respond to.
First its been said, many times, identical and in line is best, totally true, be unrealistic for most of us.
If one has a rptv you can still use a vertical center if its in the 2 ft tall range, to sit in front of a 65" screen. A smaller screen, and I assume a bit lower, maybe a shorter speaker.
If you are going to try that setup, tilt the center back, so the tweeter and midrange are firing to ear level. The sound may seem to be a bit low at times, and not at others. Kinda depends on the pitch of the voice.
One thing you can try is to have identical vertical centers, one top, one below. I tried it and it will lock the voice to the center of the screen. I just didn't like the look of a 2 ft tall center above the screen. Even laying down was too tall (15")
I do use an identical center, but those speaker's imaging is so good a phantom works equally as well. If you use a phantom and it sucks it the mains not providing a good soundstage. try repositioning them. if that doesn't work, new speakers or a center speaker is in order.
If you must use a horizontal center, use one that have the tweeter and midrange stacked vertically, or coax located.
Its been said may times before that one reason for a center is to lock the vocies to the screen for those sitting off center. true
If the dialog sucks and you have a center, I bet its a horizontal center w/o the tweeter, midrange stacked.
here is how it can work well with a RPTV
A bad center..........................................................coax LCR
Good centers
A speaker for all channels