It's not about how strict Northwest is; it's about what your hotel and car programs will do with it and that could vary by property/franchisee, whos' working the desk at the moment, etc. As others have said, it's also not about who is paying, but who is staying or driving. I have had miles credited to my account via hotel stays made by other family members, but if the hotel wanted to be strict about it, they could refuse and it is always done with the presumption that I'm one of the guests involved. It's the same with a car rental for a spouse.

Since I'm not sure if names are matched with WP numbers when mileage credits are sent to WorldPerks, I'm not sure what would happen if you simply changed the WP number for your hotel program to that of another family member. If both name and number are submitted, then the fact that these didn't match might result in a situation where NOBODY gets credited. I would be careful with this.