In general, pro players played on 3-5 on previous CoDs. Not sure where that fits into this new range of sensitivity (I guess whatever the range is between medium and high).
I think you can move it up a little higher though if you're just trying to dominate pubs. I play on 11 (the equivalent of 7 in past CoDs). You usually aren't going up against people who are good enough to make you be extremely precise from all ranges in order to win a gunfight. Plus, turning on people (you most likely won't have the chance to turn on people who are competent and can aim well, so in competitive gameplay this isn't as important) is a lot easier on higher sensitivities.