This is just like anything else RV related, it's COMMON SENSE. If the ground/driveway/garage/earth is perfectly flat or level, then having the tires off the ground is no big deal. You can flame away at this but, I've done it a few hundred times and, both me AND THE COACH, are still alive.
I say common sense because, like anything, some folks MIGHT get the impression that, if they can lift the tires off the ground when flat, and the coach doesn't move, then why not at any slope?? Well, there's no answer for Einstein's out there. Common sense must prevail in given situations. Having all 6 off the ground has worked dozens of times for me in servicing and performing maintenance and repairs. The coach HAS NOT ROLLED AWAY YET!
And yes, it does put undo strain on the air bags. I don't leave it like that for any longer than needed. If more time is needed for a given repair/service/alteration etc. and the wheels are off the ground, in comes the jack stands.
But, no one's forcing anyone here to do what they don't feel is safe or wrong. Do what you choose for your given situation.