I like doing them on the smith machine. That way you can focus on moving the weight on not on body position. The row motion should be more or less a straight line so motion of the body to conform to the smith isn't the same as squatting or benching. Besides and movement of the body will be taken up as sway between the ankles and knee. The smith also prevents you from falling over forward if you lose your balance. Just remember that you have to alter your weights due to the counterbalance of the smith machine. I take 40lbs off my weight as most smiths take up about that much. I also like the fact that you can start high then lean into the weight with out having to back away from a rack, or start low and come up into postion. Eliminating having to move while loaded is great in my book. My final point is that it is harder to great momentum on the smith as you can't swing the weight to add extra pounds.
Cheers,
Scotsman