you could have a look and ensure you are in top down camera mode eg camera looking down the Z axis. so when you rotate around it would use the camera view axis and use the perpendicular plane for rotation.
or if your in another perspective mode, when rotating you could select the axis to rotate around, so pressing R to rotate, then press Z (to select rotation axis) and then type in 72 and press Enter.
hope that helps a little bit, just learning myself.
