I still can't quite figure out how they do toe up and that is on my list of things to learn this year, though.
You can make the leg or the instep as long as you want and it doesn't mess up anything. The only truly crucial part of sock making comes when you are dividing up the stitches for the heel flap and if you leave a bit of a tail on your cast on, that's easy, it should be the middle stitch of your heel flap... after that it's all gravy, well, not gravy, but sock... gravy... Uhm... I saw a cute pattern for toe up socks
in this forum just the other day.