I don't think there is a hard-and-fast rule (or "best practice"), but, shooting from the hip, I don't think you should expand the gutter.
The general pattern in the wild is that you have discrete elastic sections that expand, and everything else maintains it's relative position. I can't seem to find a live example on the web in my quick searching (though Ethan Marcotte's seminal ALA breakdown of fluid grids does a little of it at extremely small window size). Amazon.com, Gmail, and Google News all maintain consistent margin/gutter spacing as they stretch.
"Everybody else does it" is not necessarily a reason to follow suit, but, instinctively, this feels a little more stable and consistent to users - there are less moving parts than if every element changes dimension. It's a little more self-explanatory of the mechanism, perhaps? Also, it looks less like actual browser zooming.
You could make a case for expanding them ever-so-slightly as the page width increases - certainly not in direct proportion with the width - but I doubt that subtle effect is worth the additional complexity.