It probably depends more on the airline. In about 3 weeks I'm going to Montana. My grandmother is paying for all our tickets. Well she bought everyone else's in May. But didn't buy mine because I wasn't sure if I could go. Well I could and she just bought them last week.
Well I was looking at different sites. She bought my dads and grandfather's on Orbitz in May. I don't know how much. I looked on Orbitz and the price was about $590-600. That was for a NWA flight. I decided to take a look at the NWA website. I did a search there and found the tickets were $550. About $40-50 cheaper.
So I suggest, do a search on the different sites(Orbitz, Expedia, etc...), find the cheapest flight, or the one that best suits you, and then go to that airlines website. Because if could be alot cheaper if you buy directly from the airline.