As others have mentioned, I've also seen FWC installers kick the jacks to move the camper an inch or two during the loading process.
Since I jack mine up on an asphalt driveway and sometimes leave it there for a few days at a time, I put 12"x12"x 3/4" plywood squares under the legs. That both prevents the legs from sinking into the relatively soft asphalt over time and also gives the legs a surface to slide on.
A word of warning, though. I made the mistake of 'tapping' the legs down low with a 2-pound hammer the first time I loaded the camper. Afterwards, I was surprised to see I had dented the metal of the leg. I had been watching the camper move and didn't notice that each tap was making a dent. I didn't think I was tapping hard enough to do that.
I later had a problem with the jack running off-track (and posted that here). It turns out I happened to dent the edge of the groove and that allowed the indentation to wander off track. I now have to be careful to watch for that when I use that one jack.