I don't own any treeless, so no insight there. I do mount & dismount switching sides though on every horse I ride or train, the chiro suggested it years ago but for me it's more of a training thing. I want to be able to do anything on either side of my horse.
If you face your horse's side dead on, it will pull on the saddle. If you face forward and mount perpendicular to the horse the pressure is more of a front/back pressure on the saddle than side to side. I generally mount this way as it's become habit. In general it makes getting on them the first time a lot less unsettling for the horse and doesn't put the pressure on withers or roll the saddle at all. I think going this route would be good for a treeless if the mounting from the ground statement is true.
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