In western riding you should ahve a looser rein than englihs, obviously.
HOWEVER, It depend son the horse. Obviously no horse should be congested with the reins and yanking on the face but they need to be tight enough that you can pick up without waving the reins a mile above your head. They shouldn't flap either.
IMO, There should be a slight curve in the reins to prevent the flapping with quite hands but not loose, flapping, tripping, blah blah blah.