I have often wondered the very same thing. It seems like right after I do some serious searching and then make calls to verify that the plans will work, the fares go up by a lot. However, from my experiences with nwa.com, it seems like every few days the prices jump.

If your plans are far in advance, I would just wait it out. It's always worked for me. I've even found cheaper fares within 5-10 days before I need it.

The one nice thing about NW is that they'll give you a flight credit if the price does go down before your flight. Also, they'll completely refund a purchase made online if it's within 24 hours of purchasing it. That has saved my skin several times, and it was always with no questions asked. ^