Yes they should be tightened with the wheels on the ground. After they are tightened any movement of the suspension is causing deflection of the bushes, as the metal tube of the bush is clamped by the bolt and therefore not moving. So when tightening the bolts with the wheels on the ground you make sure they is hardly any deflection in the bushes when the suspension is in it’s ‘normal’ position.

For poly bushes it doesn’t matter as the bush and the metal tube are not one piece.