My rig is a '96 Adventurer gas model with a 15', 3 foot deep, slide. It probably weighs around 1500 lbs plus the office stuff that I have there as well. The manual reads to level first - which certainly makes a lot sense to me. The larger a rig and the depth of the slide would be the only reason I can see otherwise. Some rigs are so big they could care less about the slide weight while leveling, it would seem.
Jacks are to level, but also to provide stability. Some parking sites are very uneven and might have each wheel initially at a different elevation - which would have the rig in a twisted position prior to leveling. That's OK and natural while driving, but when parked plus adding the cantilevered torquing weight of a slide on a rig that is not as yet stabilized or level while lowering jacks just seems a dumb thing to do, IMO.
Before I extend my slide, the slide and corresponding weight of the slide are directly over the two jacks on that side of the rig. If I extend the slide, the center of gravity of the side is now about 3 1/2 feet from those two jacks. So the jacks on that side have to do the extra work of lifting the cantilevered weight against an unstable coach on the other side. If I level first, it simply has to raise the extra weight of the slide straight up.