Not sure about NW, but I have read recently that AM tickets redeemed on Air Canada are booked in a supposedly miles-earning class, but I think that was an AC change, not an AM change. Plus, the person who said that also said they made sure their Aeroplan number was on the file, but no miles ever showed up.

Also, you are likely aware but just in case you aren't, you can earn Air Miles on NW transborder segments. Just link your AM number to your WP number, and you get 25 AM's for each segment from or to Canada (in addition to WP miles). Plus you can link the same AM number to more than one WP account.