When I had someone do my wheel bearings a few years ago he jacked up one side at a time with the landing gears down. I was concerned but he informed me that I had nothing to worry about since he normally leaves the jacks down. To be honest I don't think I would do this in the future. If I wanted wheel bearing (or brake) work done I'd hitch the fiver to the truck first. Common sense tells me you would be putting too much strain on one landing gear otherwise.
BTW, my fiver incurred no damage at all but I'd still be concerned.
