Tony,
We have a Lexington 255 that we just bought with two smallish slides (one bed, one sofa) with no leveling jacks. We were told by the dealer that as long as it wasn't way of of alignment we should have no problem. As an engineer, my instincts tell me that the more level you make it the less strain there is on the framing and, therefore, the motors. Most of the units with electric slides do not come with jacks because there is no built-in hydraulic system like there is on the bigger rigs. They can be added but I didn't feel that spending $4,000 or so for jacks was a good idea on a B+ rig. The larger rigs - class A units - typically have much larger slideouts and need to be leveled before the slides are used. I know a man who has a big Monaco, for example, and it comes with instructions that tell him to set the jacks and level the rig before moving the slides.