Can someone please tell me the correct range of motion for the dumbells to travel when doing either flat, incline, or decline db presses?
You will definitely get a larger ROM with dumbells that a bar, but it will obviously be in the stretch position. My rule of thumb is that you shouldn't go so far as to load the biceps. Your flexibility will probably determine your ROM for you.
Like everything else, use a full range of motion. Obviously, there are many different ways to do them. Arms out to the sides, elbows close, semi-supinated, pronated, whatever. Just do them through the whole range and no half reps.