I think most RV manufacturers recommend leveling the coach with the jacks before deploying the slides. At our stadium parking lot, where most of our local RV shows are held, however, you'll see virtually all the RVs with their slides extended, but none of them have their jacks down. That's because the stadium operator doesn't allow the jacks to be used, for fear of damaging the asphalt. Doesn't seem to hurt the RVs at all.
Here's something to consider though... and it may not apply to everyone to the same degree. We boondock a lot, where the surfaces are usually uneven, and we're nowhere near level until we use the jacks. Our coach is 43 feet long and our full-wall slide is 28 feet long. If I were to extend that slide while sitting on a very uneven surface, and then level the coach, I'd be concerned about twisting the slideout mechanism.
I wouldn't think twice about deploying our bedroom slide under the same conditions - it's only about eight feet long. I'd be somewhat concerned about our kitchen/living room slide, which is about 15 long, but that full-wall slide is long. The Owners Manual says to always use the jacks first, and then deploy the slides. So far, that's worked well.
Kev