I'd have to argue that the rule says the marker must be within the terrain piece. Not just the center.
IMO this means that where possible, the whole marker must be "inside the piece with no overlap. Granted there are examples where "within" does not mean wholly inside (Deployment from vehicles for one). Also many terrrain pieces wont accomodate the entire marker. In any case the orbital bombardment is so unreliable I wouldn't actually make my opponent adhere to this view.