My Center speaker is placed above the TV. The bottom horizontal surface is curved, and it came with some rubber cradles on which the speaker sits. You can adjust the positioning somewhat to angle the speaker up or down a bit.
I have it angled down as much as I can, and it's OK. But I'm getting a new Center. The new one has a flat and squared bottom. For those of you that also have a squared Center mounted above the TV, what do you use to angle it down? Maybe the best idea I've heard is those old timey rubber wedge door stoppers. Is there something better?