Unless you're trying to achieve some special photographic point of view, the rules of good composition require a level horizon line. When the line tilts up or down, the subject looks as though it's about to slide off the page.

Some Crop tools enable you to level and crop an image at the same time. In Elements, for example, you can drag outside one of the corner handles on your crop boundary to set the rotation angle. When you apply the crop, the program applies the transformation and trims the image canvas accordingly.