No, you don't need over hangs. They wont make any difference for protecting the walls from rain, or the damage water causes to the bottom of the wall when it hits the ground and splashes up. The advantage to having an overhang is it allows you to vent the attic space through a protected area. This is called the soffit, and it's where you will find the vents on houses and barns.
If you are going to have an attic, you need air to flow from the bottom part of the roof to the highest peak. Heat rises, and this movement keeps the wood in your roof system dry. The air flowing over the wood removes condensation and any other moisture from the air.
If you are going to have a metal roof, air flowing from the soffit to the peak will cut down, and sometimes eliminate the roof from sweating. Sweating is caused from condensation forming on the inside of the metal when the air temperature changes from the outside to the inside enough to cause the moisture in the air to solidify.
If you are going to insulate the roof, then you don't need overhangs of soffits if the insulations is against the metal roof and thick enough to stop condensation from forming.
If you are worried about stopping water from coming off your roof and hitting the side of your building, or splashing up from the ground when it hits the ground, install gutters. That splashing causes a lot of damage!!!!
For an overhang to actually protect the side of the building, it needs to extend 6 feet or more. This is also called a porch, and it's a very effective way to protect a house. You just need to do it all the way around the house for it to work.
Eddie