I use the "crop" tool as a ruler, this help to check if my verticals/horizontals are good Use one side of the selection to make a visual straight line and you will see if something is wrong in the image. Just escape after
One thing that also helps to straighten a pano is to rotate a little bit the crop tool selection to make the a slight inclination. Then you'll have to hit enter to validate.
The idea of a moving ruler or a cross-hair in APP would be nice
Well, what I see here is just the standard Photoshop / Gimp crop tool - select area and rotate selection. Surely a nice thing to have; nothing mission-critical though, IMO. Knowing how things develop in the beta cycle, I won't be surprised to see this in a 1.4 betaX