"FIRE UP"......Here's why the old Monaco recommended their procedure. First, they used a three jack (point) leveling system. Second, Monaco's chassis would be considered a monocoque chassis. When Monaco installed their slides, the coach was fully aired up at ride height. Their slide openings were square. Even though a campground site may not be level, the Monaco chassis would sit pretty rigid, disregarding the unevenness while aired up. Consequently, the slide openings were still square. You deploy the slides, dump the air and then level.
Many will argue this procedure to ad nauseam, but it worked for the Monaco coaches for many years. Newmar recently changed their procedure on their full wall slides to extending first and leveling second. On my FWS, I determine the level of the site before I extend. If the site is fairly level, I level first. If the site is not very level, I deploy my FWS first and then level.