briggidere - 12:19 am on Oct 5, 2010 (gmt 0)
Be careful with position preference, it can dramatically reduce the overall impressions of your campaigns. If you choose a certain position and AdWords decides you don't qualify for the target position in an auction it can just not bother to show your ad at all.
Taken from the link provided by RhinoFish
Finally, let us note that setting a position preference can sharply reduce the number of impressions and clicks you receive for that keyword. Targeting just one or two positions means your ad will not show at times when it otherwise might have. We encourage you to choose as broad a range of positions as you are comfortable with.