A lot of this depends on what kind of target you're trying to hit. Remember, barrels in almost all pistols and rifles are angled up relative to the ground (not parallel to the ground). This is what causes the bullet to rise up and intersect with your gun's sights at whatever distance you are zeroed. If you turn the gun 90 degrees to the left, now your barrel is tilted to the left. The bullet is also not going to climb at all like this, it will begin falling to the ground as soon as it leaves the barrel. The end result may be that you notice that your impact is low of where your point of aim was. I've seen this as much as 3-4" low at 25 yds when shooting canted around a barricade. For low percentage shots like this or trying to hit the upper A box of a USPSA target, I try to keep the gun as perpendicular as I can.