When no threat is detected or you are just simply walking back to respawn, you need to keep your gun pointed up. This will help avoid accidents and keep people from shooting at you. This picture has a little exaggeration but you get the idea.
Step 2: Getting Closer To Contact
When you hear gunfire in the distance or get word of enemies a ways out, then you need to hold your gun almost in the ready position, but not quite all the way. This allows you to quickly go from prepared to firing. This picture will help you understand the main idea.
Step 3: Enemy Spotted
This is when you see an enemy and are ready to fire. If you are within 50 feet of contact, you should always have your gun assuming this position. Just like this picture.