I learned long ago to avoid ever giving an account number unless it is definitely the one that you want credited. I fly Delta a lot and always credit it to my NW account (for the elite bonus) even though I have a Skymiles account that all my credit card points go into. So every Delta ticket I buy, I have to make sure that I am not signed into my Skymiles account, and then I have to remember to add my NW number later, either at check-in or by phone. Yes, it takes longer to enter all the information each time. But it is the only way to make sure that Delta doesn't screw up the miles.