A thread over at Cre8asite named Deciding What Not To Track / Measure discusses an interesting question. What metrics should be tracked and which ones should not? The thread creator, who I respect, says "what you choose not to track can play as great a role in creating a successful online presence as what you choose to measure."
I agree with that statement, but only if you look at that statement the other way around. Its like the story of the ostridge bird hiding its head in the sand when its scared. The ostridge feels that what it doesn't see can't hurt him, but in reality, the environment around the bird can be very dangerous even though one is unaware of ones surroundings. Thought it would be cool to include a picture of an ostridge now, brought to you by terra.com.
I am of the opinion that one should track every metric possible for a particular site. Of course, if your tracking tools have a negative affect on site performance (speed) then be smart about it. Even if you do not want to look at a particular metric today, you might want to look at it in a week or a month. This way, if need be, you can be prepared for the environment around you.