Now that I pay attention while I'm scrolling I have my pointer finger on the left click, middle finger on right click, and I scroll with the side of my pointer finger.
It's just uncomfortable having my middle finger on the scroller.
Either my middle finger or the page down key, whether with a mouse or a touchpad, it just feels more natural for me to use the middle finger, keeping the index free for highlighting/clicking.
I suppose it depends on what kind of mouse you use. I use a pebble shaped Apple Mighty Mouse (with the dinky scroll ball rather than a old fashioned wheel) I think it would be uncomfortable to use any other finger than your index finger with this kind of mouse. I just don't have the dexterity in my middle finger to scroll the ball left and right as well as up and down with the same accuracy.
Index finger on a scroll wheel. Middle finger to move around on a trackpad, so on PCs, middle finger to use the scrolly things, but middle finger and ring finger to scroll on a Mac and PCs that support multitouch.
I alternate hands when using the trackpad though (which I use most of the time). In which case, I use the middle finger on my right and the index finger on my left.