If it is a stage bottle you would be breathing it first, so it would get light before the main tanks, so would not be hanging down by the time you were moving it and the main tanks down toward your crotch D ring. If you are talking about deco bottles, then yes, it would be foolish to clip a full bottle below a light tank because it would hang low. In that case I would actually top-mount the deco bottle(s) so they help hold down the main tanks as they get light. If you are not comfortable with that, then bottom mount the deco but leave it clipped out back on the drop ring or butt plate and just move the main tanks forward as they get light so they pull right down onto the deco.This is it, ali's end up nearly clipped to my crotch D'ring by the end of the dive. If a stage is clipped there, it will just dangle under you witch kind of defeats the purpose of sidemount.