Thanks everyone for the feedback.
I have it in front now, with my TV on a piece of plywood with spaghetti sauce cans underneath to elevate the TV a few inches so that the bottom of the screen is not blocked by the speaker.
I don't currently have any way to put the speaker above the TV for a test-listen. I need to decide whether to build a support for the speaker to be above the screen, or a more permanent TV riser for speaker-in-front placement. Maybe I can rig something up to test the above-screen placement, hmm... what could I find on sale at the grocery store for that? (jk)
The height of the center channel tweeter is about 12" below the mains with the speaker below the screen, and about 6" above the mains with the speaker above the TV. Advantage: above.
The center baffle is about in the same plane as the mains when placed in front of the TV, and it would be in an arc when placed above the TV. Advantage: above. But I can set the distance in my receiver setup to compensate, as 4DHD points out.
I suspected the "line it up with the screen" advice was mainly about reflections off the top of the TV. Thank you for the confirmation. If placed above, I can have the speaker flush with the screen, but I can't really move the speaker more forward than that (at least not with the stand I was considering making). It seems like putting the center in front of the TV gives the least amount of stuff for reflections. For example, the front edge of the TV stand would be in front of the TV/speaker (even if it is 2 feet down) if the speaker is placed on top of the TV. I suspect having the speaker in front is better for 2-channel listening, too, since the TV and center speaker baffle won't be in the same plane. Advantage: in front.
Conclusion: I'm going to build a better riser for the TV and keep the center in front.