Like most replies, I also typically rig tail down, but sometimes I switch things up... it all depends on what I am trying to accomplish.
The bottom line is the position of the curl in the tail makes a difference in how the bait runs. As the tail stretches out in the water, it (in essence) forms a wedge, which will cause the bait to track in the direction opposite the curl. Every curl tail possesses this effect and the bigger the tail, the more effect; this can be seen by rigging such a tail sideways on a disproportionately undersized lead head. (Conversely, a small plastic tail on a big lead head will have minimal effect.) This can be beneficial knowledge in a couple of instances:
>Many of my walleye friends carry lead heads in 1/16oz & 1/32oz divisions, but I skip those tiny weight differences (staying with 1/8oz marks) and adjust the speed at which my jigs fall by switching the tail position up or down.
>A bigger grub (like Kalin's Mogambo), can be rigged sideways to counter the tendency of a buzz bait to track to the side (without bending the wire) or it can be rigged tail down to allow most buzz baits to stay on the surface with slower presentations than a skirt would facilitate. (Similar effects can be applied to spinners baits as well.)