I start with this image:
As you can see the horizon is not entirely the way it should be.
To solve this, I use the crop tool (R) in Lightroom (in the develop module).
There's a few options to straighten the horizon. The first one is to click on the Auto button in the middle of this section.
Click Done and voila, the horizon is straightened.
The second option is to do it yourself by using the straighten tool.
Click on the icon and then draw a line over the horizon.
It's easy enough to correct it in post processing. But why not make it even easier and pay attention when shooting. After all, when you have to straighten it in Lightroom you will end up losing some parts of your picture.