Consider the shape of your crack dealer's average target. It tends to be a person and people tend to be alligned vertically when out on the street.

Consider the direction of movement of a handgun when it is held in the correct manner. It tends to tilt backward, moving the line of fire vertically upwards over your target. Gravity will then help to bring the gun down again, lining you up nicely for a second shot.

Now, if you want to look cool you might like to hold your gun sideways, but please be aware that any recoil will force your weapon away from your target and there will be no helpful universal force to bring it back. You had better hit the sucker first time if you want to look this stylish more than once.