I just went and looked at some I printed, and the half dozen from shows hanging on a doorknob.
They vary from pretty much centered, to a little higher than centered.
Same rules as shirts, in that sometimes one needs to all but ignore some incidental part of a design in favor to a dominating shape.
Thing big elipse or circle logo with a small tag line. I noticed that on most of the giveaways, only the main logo was centered, while the line dropped lower.