The topic of level rig and then slide, or slide and then level was "hotly" debated on another large RV forum site (multiple times).
The results were (are):
RVs with Full Wall Slides (FWS) are, because of their longer slide length more picky that the chassis is not torqued (twisted) before the slides are moved. This is because the longer length of the slide magnifies any amount of twist in the chassis. It is also because the RV has less structure due to the larger opening that lends itself to less resistance to twist. The manufacturers have added more geometric structure in the floor to account for the reduction in box structure.
Several manufacturers state that before the slides are moved the operator should go outside and visually inspect that the gaps between the slide and the chassis are even side to side. If that is so, then if the RV is somewhat level, then the slides can be moved. If not even, then using the leveling system, level the rig and visually inspect again. This is really steps to verify that the rig is not just level, but also not twisted.
My hypothesis is that many rigs don't have leveling sensors at the front AND at the rear of the vehicle to properly detect twist, so the operator must visually inspect first. I am working on a solution for that problem.
The high recommendation from most is "follow your owner's manual for the procedure".
My personal observation is the RV should be level and square (not twisted) before moving slides. If the vehicle is at that state sitting fully on it's wheels, fine. If it is on levelers in that state, fine.
And, if you have FWS - you should be even more wary that it's in that state.