Hi Bob, I don’t have slides so I can only speak from logical thinking.
The idea about not deploying the slides before leveling is most likely because of the body of the RV being tweaked so that slides’ tracks might be interfered with from the torsion.
The same thing goes with the door. If you are parked in a not very level area, you’ll notice your door may feel tighter or stickier when opening it.
If the ground is not level, it would probably be a good idea to level your RV before deploying slides. I would also tend to think that always deploying slides after the jacks is better because the slides would be encased for the possible extra-torsion that leveling would cause. That way you know that whatever path the slides slid out on is the same they’ll come back on.
Deploying slides before the jacks might mean they slide out on tracks slightly skewed one way and then retract on a different skewed angle. This may or may not cause some damage down the road with the extra wear. Not just on the tracks and rollers, but on seams and edges.