My understanding is that CO now makes you pay the change fee, so unless the drop > change fee, it's not worth it. For most flights, the change fee is $100, but on some tickets it may be less (particularly if CO competes against B6 or WN on the route). It never hurts to call and ask.

Note that AFAIK, NW will still issue a voucher for the difference minus a flat $25 service fee if the price drops. So if the price drops on an NW ticket by more than $25, you should definitely call and ask.

Of course all of this applies only to nonrefundable tickets (obviously).