NW charges the least FF fee on National too. It's just pennies on a contract. Looks like a great promotion.
If you want to make sure your miles will be credited on your contract but you had someone else book your rental, go to Nationalcar.com or Emeraldclub.com and click, existing reservations. Enter your res number. Find out if your NW ff number is listed. If not, enter it and update your res. It will adjust your total price very slightly.
(Most rental car companies have always had to pay to credit your FF miles and those fees have been rising. So now the tax on those miles is getting passed on to the ones who are getting the benefit, YOU. NW's ff tax is one of the lowest.)
So anyhow...before you set out on your trip, that's how you can make sure you will get your miles credited on your rental. Then you won't have to remember to tell your rental agent to do it at time of rental or your gate agent to do it. No missed miles.