AI SERVO, Center Point, Positioning Question

I shoot youth soccer with a 7D, 300 f4 and 70-200 f2.8.

I always use AI SERVO with the center point selected.

My question is this.

If you are trying to frame your shot so that the subject fills the viewfinder, do you leave the center focus point in the middle or do you move it up so that you can focus on the head instead of the body?

I guess this would apply more to vertical shooting, rather than horizontal.

Thanks...Alan