For I long time I played on 5 for H3, then switched to 4 for about a year, and a month or two ago I just switched down too 3 and my aim has never felt smoother.
it's heavily dependent on the size of your tv. I had a 24" tv and played on 5, with a really smooth BR. Now that I have a 32" TV its somewhere around a 3.5, 3 is a little to slow and 4 is a little bit jumpy, but i've been having decent results with 4.
I think there is something to the theory that you need a higher sensitivity on a bigger screen. I just recently moved to playing on a 52" and I notice that I have a hard time "sweeping" across at a fast pace. I have always played on default, but I might need to turn it up.