That all depends on your comfort with your device and if people are going to mess with it. I leave ours signed in as only my employees can access them. I don't see much risk other than a person ringing up fake charges or possibly refunding transactions, but that would assume they have knowledge of square and are for some reason targeting you to refund your transactions.
If you want to sign out it doesn't hurt anything other than taking a few seconds more to sign in before you need to do a transaction.
