My HWH system has a dump air button so I can dump with the engine running or not. If I am boondocking I usually leave the engine running while I deploy the jacks. If I am going to plug in, it's not much of an issue.
My manual also recommends deploying jacks and being level before extending a slide room (as did a previous 5ver and TT). It is the only thing that makes sense to me. These MH chassis are rigid but will still flex and twist if you open a big hole in the wall. If the MH happens to be dipped or sloped one direction or the other, the added stress on the frame by pushing out one or more slides multiplies the stresses that already exist. This can cause slides becoming out of adjustment, or if your slide has a track with teeth on it, cause the gears to jump a tooth on the rail.
Just my .02, but it works for me