When you are in Travel Mode the air bags have the coach body square with the frame, and there is no twist in the body at that point, even if you are leaning one way or another due to unlevel ground. That means the slide out rooms are also square all-around with their openings in the body. That is the time to put them out or take them in - when you are in Travel Mode.
If you level the coach on uneven ground, then by definition the leveling action has to twist the body in one way or another. Moving a square slide through a body opening that isn't square ain't a good idea, and something's gonna get strained needlessly. And as Larry said, if you get leveled up and then put out slides, the weight of the extended slides will torque the body and throw off your nice neat level. So you have two very good reasons to burn into your mind the old Monaco adage, "Slides out first and in last".
My wife remembers it better than I do, and catches me on the rare occasion I try to level while I'm still in the driver's seat after parking - an easy thing to do! :X So get your co-pilot to memorize the adage too.
-Joel