[Ifeffit] choosing dk values

Hi all, I'm doing some fitting with Artemis, and I noticed that, for different values of dk, I get better or worse fits. The only information I've gotten so far about dk was in the IFEFFIT documentation, and it basically said, "Don't mess with it." Can anyone give me more information on choosing a good dk value? Or, should I just stick with the default? Thanks, Mike Groves