Placement of Center Channel Speaker

Everything is "balance" ultimately!...and relative to your set up, room, situation.
Placing it up high will shift the soundstage relative to mains, and will couple differently in the room, so you gotta tinker basically.
I think most would be happy this way, but depends on your desired results.
I tend to put center speakers where they best anchor the sound to the picture, and sonically SOUND THEIR BEST!...IF not the same as the left, right, and other speakers!
Your goal is to make the center sound like the mains, and vice versa. And speaker placment has a tremendous amount to do with that.
So, in short, placing the center (often compromises) where it yeilds similar(preferably "flat response") frequency response to the mains is ideal.
Also to consider is the often "chesty" sound added to the cneter when placed on top of a tv. So, tweaking and issolation often becomes necessary.
OTher consdierations are that the speaker is laying down on the TV mostly, unlike the other speakers, which changes dispersion often. So, it's a balance.
Try this...place the center speaker in your chair at "ear level" for the woofers, and play some music with bass, then walk around the front of the room/TV and see where it sounds' the best! This is a neat trick. Otherwise, sound level meter and test tones, or some sort of method to get you to get the best response from the speaker.
Aiming the speaker will effect tonality and dialog, yes.
So again, wherever ultimately the speaker sounds the best, is where it goes...assuming it flows with the rest of the soundstage, provides proper perspective and steering, and integrates to your liking. Many don't like sounds going from low to high to low agian across the front, so try.
Still, most won't care.
good luck

DSK and "infinity audio", for the most part, an easy wasy to improve on the sound of a speaker placed on top of a TV is to "de-couple" it a bit, by using "isolation platforms"(Lovan's "tri-solater is one), which many companies offer! You can also put points, and other issolation devices between the speaker and the TV, but I think more solid is better. Simply laying a speaker on top of a TV gives too much dirrect contact and surface area with which to transfer vibrations from the speaker cabinet to the TV cabinet! Yes, a less than stout TV box will take sounds from the system itself, and reasonate them into the TV, ading sound to the room, which isn't desireable. But, it's small compared to having a speaker laying up against a set, which magnifies the problem!
Anything really(even "sticky tack", or "blue tack" between your speaker and your bigscreen will help somewhat, and also help anchor your speaker for sound and stability. But, I think it's better to issolate them even more.
Anyway, that's probably the biggest thing you can do to issolate sound from mixing between your speakers and your TV! Note: your center speaker and your TV, when mixed together, become one larger speaker resonator!
Just lay against someones chest while you're talking, and hear your sound echoing through their body some time!...it's the same kind of issue.