I seem to finish mostly vintage and antique quilts, so I tend to make narrower bindings - 2" or 2¼" folded in half. They're about 3/8" or a little less when they're finished.
It just depends upon the look you want, though. Some quilts look better with wide bindings - puffy comforter-type quilts are so sweet with big puffy bindings.
I haven't tried a micro-binding, yet - a recent magazine had instructions for putting 1/8" bindings on miniatures! Yikes - that's teeeeeeeeny! Might be fun to try it. :)