I tape and bed all joints whether inside/outside joints and butt joints.
I like to think that the tape whether paper or my favorite fiberglass mesh, will adhere the sheets together better and work against flexing and thus cracking.
Mud alone will crack and give way with any movement of the frame or house. Paper and especially mesh will give to an extent.
Now, you can get that special folded paper tape for the inside and outside corners. But I would strongly recomend using the metal corners for all outside corners, they protect the edges so much better than without.