don't measure your gauge in dim-lit air force hotels. you might measure 9 stitches per inch and not realize till after you've knit the heel on your toe up sock and tried it on, that hmmm, the sock is a bit large, and after re-measuring in your well-lit living room discover you're actually knitting 8 stitches per inch. which means you didn't have to muck about with the pattern either.
of course the genius of toe-up socks is supposed to be that you can try them on as you go, but i can't really tell if the foot fits proper until after the heel myself. unfortunately.