Always call NW in the USA to get seats allocated for a NW flight! - Never, never get KL to do it - the last time I tried to get the UK res callcentre to get me decent seats on a NW flight, they said they'd have to email Amsterdam (!) to get authorisation......

And do check that NW have the KLM PE number in their system - sometimes it transfers, sometimes it doesn't. The NW res agents seem to be much better trained and to have much more authority than their KL counterparts (who don't appear to be allowed to do anything at all.)