For true dialog speakers, I think it's best to have the television immediately above or below the center speaker. It would be best to have the center channel immediately behind the television, which is possible with an acoustically transparent screen. But I doubt many people are running projectors so that's rarely an option. Short of having an acoustically transparent screen with speakers behind it, I think it's best to have the television right above the dialog speaker. When the midrange/tweeter is just under the televison, the illusion works pretty well.

If the room isn't wide and the right and left channel flank the television anyway, I don't think there's much to be gained by having a discrete center channel. A virtual center is fine in that case. I run a pair of speakers flanking the television to create a virtual center. That's a good solution in my living room; There's a wide soundstange and definitely no trace of any missing area between them.