My 2007 Monaco Diplomat Owner's Manual says to extend the Slides then level. When I asked Monaco they said that leveling puts some stress on the Frame and might bind the slides.
I look at the lot and estimate how much its going to take to level and then make the decision about when to extend the Slides. I usually put my Slides out after I have dumped the Air, leveled and made my power, water and waste connections (lot easier). On departure we put the slides in first, disconnect power, water and waste then start engine and raise the Jacks, that way I only have to run the engine once as the Air builds up in about the same time it takes to raise the Jacks.
My BIL's 2004 Winnebago Adventurer Owner's Manual said to level first and then extend the Slides; however, his large slide would not extend after leveling on many occasions. He would have to lift the Jacks, extend the Slide then re-level.