I think your guitar should ideally be quite high up when you're standing up. It should feel as though you're sitting down with the guitar. Know how you feel so comfortable sitting with your guitar? Well that's how it should feel standing up. The more comfortable you are, the easier you can access higher notes and pull off more intricate stuff. If your guitar hangs too low, you won't be able to alternate pick properly with your picking hand and your fretting will be sloppy.