Unless you're deliberately doing something artsy-fartsy, please keep your horizons horizontal. When I see land/sky or sea/sky horizons even 1/2 a degree off level, it drives me nuts, primarily because there are two EASY ways to fix a misaligned horizon.
- In camera (when you're shooting it) - Some cameras' LCDs even display the grid for the rule of thirds (my el cheapo DSLR does!). Just line it up to the bottom or top horizontal line on the grid and voilà: instant composition AND a level horizon. Wow. They just couldn't make that easier for you.
- In Camera Raw (Photoshop/Bridge) - In case you managed to screw it up while shooting, Camera Raw has a miraculous tool known as the "Straighten Tool." Just use that sucker to line up two points on the horizon and you'll look like a pro.
Now, I expect no more lopsided photos. ;)