[wavemachine]

To me it is what looks best, some of the shots I have taken I have experienced what you have mentioned however straightening the horizon can ruin the rest of the image. Trouble is that some people will just pick up on an un-level horizon without looking at the rest of the image. Also it is not like you can say in a competition yeah the horizon was naturally un-level here it is with the horizon straightened.

Having said that if the judges of a competition are worth their salt and the image is well composed they should be able to tell the horizon is naturaly unlevel by looking at the rest of the image.

If you shoot on a tripod with a spirit level in your hot shoe at least you can be confident the camera was level when you took the shot!