IMO, it's best to buy early if you are not aquainted with the route you are taking. Living here in South Florida, I would never buy a ticket to the Northeast or West Coast unless it is a really good deal (under $200 R/T to NYC/WAS and under $350 R/T to LAX/SFO). Since we have 3 major airports here, someone is generally having a sale. If not, just wait a few weeks and someone will.

On the other hand, if I was flying to Boise or Witchita Falls, I would probably buy it as early as I could.