Well one reason I am definitely upgrading to the iOS7s software is because I can't wait to use it! I never had a beta so I am super stoked!
Another reason I think upgrading would be good before you get the iPhone 5s is to get to know the software and update your phone to look how you want it to look. Then you back it up with the new software and when you get your new iPhone 5s all you have to do is put your backup with the new iOS7 software on the new iPhone 5s and it's like you never lost a beat.
If you're starting from scratch and never use your backups then I suppose the backing up wouldn't matter to you. However, you could still download the new software and play with it just to get a feel of it before you get your new iPhone 5s.