In my experience, the $25 per ticket fee has been deducted from the amount of the fare decrease, and an MCO has been issued for the difference. For example, if the fare goes down $50, I get an MCO for only $25, after the "service charge" has been deducted. But this fee is relatively new....used to just get the entire amount back as an MCO which I would apply to my next ticket purchase.
All of the above is the way my Travel Agent handles these things. Don't know how NW handles them if you have bought the ticket directly from them. And Lord help you trying to get an MCO back from Expedia!!!