I wouldn't call it unsafe for most designs, but it could negatively affect your shooting.
"Cast off" refers to when the butt stock is bent slightly toward the outside shoulder (bent to the right for right hand shooters, as you look down the barrel from the breech; to the left for left hand shooters). "Cast on" means bent towards the center of the chest.
Good shooting of rifles requires accurate aiming which, in turn, necessitates getting your eye squarely behind the sights (and not off to the side even slightly). When the stock is cast off, it's easier to get your eye squarely behind the sights, as the breech bends slightly toward your head. One doesn't 'aim' a shotgun so much as 'point', but pointing is also improved by correct eye placement.
Some gun stocks have no cast and are no advantage to left or right-handed shooters.
Do you have a line on a left hand model shotgun? I'm in the market for a left-handed trap gun for my daughter. So, in the unlikely event that you've found one, perhaps I can buy it and find you a right-handed one (assuming you're right handed).

