I don't think it stresses their back too much more as long as you don't slam down into the saddle when you throw your leg over. That applies whether the horse is parked out or not. I always cringe when I see some people mount up and come down in the saddle like a ton of bricks and their horses backs give like a suspension bridge in a gale force wind. It hurts me just watching it so I can imaging how their horses feel.
I come down really soft. I'm just working on getting on faster so I don't stand in the stirup or drag my right leg over.