Try as I may, I cannot get straight horizons, so I feel your pain. Shooting off a monopod with the 300, I always seem to wind up with a crooked horizon. And I am aware and try to get it straight. It is so easy to adjust the horizon in Lightroom, that it has become a natural thing for me when looking at each image...check the horizon, crop for composition, adjust exposure, sharpness and noise reduction, etc. Some of it is done in a batch operation at the beginning of each set of images.
Goog luck and keep on shootin'.

