Hi Robert,
That will depend upon the competition. Some do not allow any photoshopping whatsoever:
http://the-photo-news.com/photograph...photoshopping/
Some may well allow it. Some also will expect it. In the end, it is up to you. You may deliberately choose to slant the picture, and that is part of the art of photography. If you wanted it that way when you took it, submit it that way. At the very least, it will stand out from all of the other one's with perfectly straight horizons.
Frank