PowerSteering.com's practice is to ship gearboxes on center. And as the heart of your steering system, its center position is important during installation. However its not difficult to tell if a gearbox is on center. The majority of steering gearbox brands, power & manual, have gaps in the pitman shaft splines that tell you if it's at center; the input shaft provides a clue too.
Most pitman shafts are indexed with 4 large spline gaps. When the spline gaps line up with the axis of the steering box, its on center. If they aren't lined up, it's not on center.
Furthermore, when at center the flat on the input shaft will be at top-dead-center. If there is a rag joint attached, the rag joint's pinch bolt will be at top-dead-center. See the photos below for examples. The input shaft alone doesn't tell you if you're at center, i.e. if its a 4 turn lock-to-lock gearbox it will be at top-dead-center in 4 places. (These conditions do not all apply to reverse rotation or 4 x 4 GM gearboxes.)