Whichever spot makes it closer to ear level is the best spot for it. If your screen is acoustically transparent, then you can place the centre speaker behind it to make it exactly ear level, which might possibly increase the WAF.
Personally, I prefer to have the centre at a little bit above ear level and tilted down a bit. That way, people who are in the second row can still hear the dialogue easily (as opposed to having the centre too low).
cheers,
supervij