In Photoshop/Illustrator/etc it's common to move an image along only the X or Y axis if SHIFT is being held down. When I edit a Panorama, a frequent adjustment I make is just to pan the image left or right to adjust the composition. Without being able to lock the axis though I usually end up with an wavy panorama that takes additionally nudges to get everything back into a straight line again. It's be much faster to grab one side, hold down SHIFT then drag left or right so that the wave effect doesn't occur.
Try setting the grid to display in the Editor and then use the centre point tool to pan the image along the horizon line.