I have to admit that I never used to straighten my photos and thought they were fine the way they were. I was actually quite annoyed once when someone I didn’t know left me a comment on a photo on a photo sharing site saying that it would be improved if I just straightened the subject in it. Actually at that point in time I didn’t even know about straightening tools (so maybe that was why I was annoyed). I have since discovered at almost every photo editing program out there has a straightening tool in it and the majority of them come with grids so you can easily see if your subject is straight or not. Crooked shots have now become one of my major pet peeves, that is unless it is intentionally crooked, just be careful with this as it can be overdone. You do not want your viewers getting a kink in their necks trying to view your photos. A slant tilt on some shots can enhance the right shot though.
Please check your horizons and make sure they are straight in your landscapes.