I don't get too excited if the rear wheels come off the ground because unless the jacks are particularly flimsy, or they are resting on very smooth concrete or ice and the slope is considerable, the jacks are going to resist any sideways movement probably better than when people are deluding themselves that they have braking just because the tyres are still touching the ground even though there might only be a few pounds of weight on the tyres.
Always better to lift the front wheels off the ground but sometimes the direction of the best views require the rear to be higher.
Actually there are few occasions boondocking when the ground is so level that the MH can be levelled without lifting at least one wheel off the ground
International Grey Nomad.
Travel photos at Picasa Album
Trip map at Spotwalla