Tab-targetting or just auto-targetting has a couple specific rules to it that most players are not aware and most never will be.However, the more targeting you can do w/ your keyboard instead of your mouse the better.

How it Works
When you use your targetting function, the first thing the camera checks is the 90 degree arc in front of your character. The closest mob within a 40 yard range becomes your target. With tab-targetting, it cycles through different targets in this field and will generally stay within the field.

The second place the camera checks is for any mobs within 10 yards of your player in the other 270 degrees. These will normally be cycled only after all targets in the 90 degree arc have been cycled first.

Why is this important?
Basically, if you want to maximize your targetting, especially quickly between targets, you want to use your keyboard to do as much targetting as possible.

How does the mouse factor in? Simple; the mouse is still the most important targetting tool. You use the mouse to rapidly angle your character in the direction he or she needs to be to snag the targets you want.

By doing this, you are actually faster than individually targetting mobs w/ your mouse, because you do less actual movement and require less precision.