I actually have mine centered between 2 of my monitors, but angled slightly to the right. In other words, the right corner is closer to the edge of the desk than the left corner.
I have 4 monitors hooked up in my office, but find that I use the 2 on the right the most frequently. Hence the slight angle on my keyboard.
Also, being a UNIX person, I tend to rarely use the mouse anyway, and when I am, it's just the mouse. So I tend to move the keyboard over to accommodate the mouse when I need it.