Shooting with your camera plumb-n-level, that is, keeping the vertical and horizontal lines in your viewfinder plumb and/or level, is often a good idea. Shooting a pretty, bikini-clad girl at the beach is a good example: If the ocean's horizon is sloping, even a small amount, it is often not a good thing. In fact, it can be downright distracting and mess up an otherwise good capture.
Yeah, you can make leveling adjustments when cropping but, sometimes, that inhibits other ways you might want to crop. My advice: Better to keep things plumb/level when you shoot--leastwise, if that's what you're looking for--in order to make your work in post go a bit more smoothly. READ MORE?
(Caution: Link contains NSFW pic)