here are some jpegs as an example at either end of the focal length scale for a particular zoom lens. Barrel distortion at wide angle, pincushion at telephoto (not so obvious).
(hopefully I'm not infringing on dpreview.com copyright but this is purely for teaching purposes)
All lenses have some level of distortion and this can either enhance the image or be disconcerting to the viewer.
Personally I like a bit of barrel distortion especially if the horizon is water. The earth is round after all and the curvature in the image (although an artifact of the lens) gives the sense of vastness. Its about personal preference. Its also about working with the limitations of your gear and working to its strengths.
The only "straightening" I would do would be to make sure the points where the horizon touches the edge of the frame is at the same height on either side....or not. Its up to you.
