David, please be certain to fact check before making declarations purported to be true.
Also, when you jump to a conclusion, as you do above, please identify it as that - that is, nothing more than your opinion (as opposed to what you expect readers to believe as an actual manufacturer "recommendation", which it is not.)
Moreover, if you seek to leverage your claimed experience as a credible contributor to AV publications, then as readers we expect that when referencing speaker set up recommendations from any manufacturer, you will please follow standard and expected journalism rules of fact gathering and reporting by providing a cite to the so-called "recommendations" upon which your argument relies. We are smart enough to ferret through the source material and come to our own conclusions.
THX's website does NOT make ANY recommendations for preferred placement of center channels or any other channels. THX clearly states that the center channel can be placed "either above or below the display".
http://www.thx.com/consumer/home-entertainment/home-theater/surround-sound-speaker-set-up/. Curiously (to me at least), their artwork diagram for speaker placement shows the center channel above the display.
Stewart Filmscreen's website does NOT make ANY recommendations for placement of center channels or any other channels.
http://www.stewartfilmscreen.com/. However, one MIGHT be tempted to conclude that either above or below screen would be okay given that they have multiple photographs of installations where both center channel above/below locations are featured. But again, that's an unfounded assumption and would need to be verified with Stewart Filmscreen before asserting it as their "official" or recommended position.
Dolby's website does NOT make ANY recommendation for preferred placement of the center channel beyond stating to "keep [speakers] at your ear height when seated" but also advises that,
"The configurations noted in this guide are illustrative." http://www.dolby.com/us/en/guide/surround-sound-speaker-setup/index.html.
Additionally, there is NO statement appearing on THX, Dolby, and/or StewartFilmscreen's websites that "tweeters of L-C-R should be as in-line as possible." In fact, one could make the argument, based on the images of installations all of those manufacturer use on their sites, that the opposite is true; that is, that tweeters of L-C-R should NOT be as in-line as possible.
My installation involves use in a dedicated theater with three levels of stadium seating where I project onto a 128" screen. A Klipsch RC-64 II is installed above-screen in a 7.1 set up. A Yamaha RX-A3010 combined with a single channel power amp rated at 250 watts to that center speaker has worked as well as I or anyone else could expect. I don't need to go into the details of the rest of the set up, but as in ALL cases, as most of you already know, one's mileage on center channel (and others) preferred placement will vary and depends on so many other factors outside of just asking the OP's question.