Bruce,
There will be people that will tell you "never lift the rear tire(s) off the ground". If you are on solid ground, you are transferring the weight from the tires, to the leveling jacks, (I assume that your jacks go straight up and down). If the ground is unstable, why would you have your MH on it? Most leveling jacks are bolted/welded to the actual chassis frame, and are very strong. I know that dealers put the slides out without leveling, and a lot of time, in very unleveled areas. You can put a lot of stress on the slide mechanism if the MH is not level. You can get away with it, for a while, but you are asking for trouble doing it. Use common sense here when leveling your MH. There have been many Pay RV parks where we had to raise the rear wheels off the ground, and a few RV Resorts. We have had a lot less trouble leveling boondocking, as there is no real restrictions on where you can park, so leveling is easier.