Well, it depends. This is a good fare, and you don't want to lose it.

My recommendation is to use Telephone reservations.

If you "buy the ticket" with NW telephone reservations and give them your credit card, you guarantee the fare until "midnight tomorrow" central time. Your ticket will be issued after that. So, you have until it's issued to cancel or change things.

If you "buy the ticket" with nwa.com, your ticket will be issued almost immediately. Then, should you wish to change or cancel, you must do so by "midnight tomorrow" and go through the refund/exchange process (7-10 business days, and more of a hassle).

Since these two avenues vary in how they process tickets, go with telephone reservations, as it's the one that would be less of a headache if you need to back out.