These are all excellent suggestions and tips, but what I would like to see most is the ability to easily move the grid while working with perspective corrections. I.e. translate it horizontally or vertically. I figure this could be controlled by the arrow keys in combination with one or two modifier keys.
When making adjustments for buildings and similar objects, it is often useful to have a gridline on or a few pixels away from a line that should be vertical or horizontal. Switching to a finer grid does not help but rather makes adjustments more difficult, as it obscures the perception of straightness of lines in the image, which are frequently relatively low-contrast. Adjustments are an iterative process, with changes to “vertical,” “horizontal,” and “rotate.” One checks the correctness of a line to the right of the image, does some corrections, checks a line to the left, makes additional corrections, and maybe if the buildings themselves are not perfectly straight, or distortion correction is not perfect, one checks lines in the middle, too, in order to find a compromise.
When one is close to the optimum, it would help a lot if, at each correction step, one could quickly and easily move the entire grid some pixels left/right or up/down. I am not talking about any kind of automatic movement. Just an ability to manually move the grid when desired. Preferably with keyboard shortcuts.