If you receive personal emails on your phone then you are using it as a sensitive data access point. Unless you don't mind allowing someone who finds your phone to have access to a treasure trove of data. Contacts, photos, your home address, your personal calendar (that instantly tells when you'll be away from home), etc. Within minutes, long before you can get to a computer to change your email password a mysterious evil-do'er could find out just about everything about you. An email keyword search only takes a few seconds. They could probably beat you to your home to clean you out! OK... OK... That's a bit over-dramatic, I know. But even something as seemingly innocuous as private email is pretty sensitive and you should lock your phone. Unless you don't receive emails to your smartphone, and if you don't do you really need one?
But hey, that's your business. Do what you like.