yes, a grid would help... but a bubble level helps more, especially when the horizon when lines aren't level with horizon, or they are at a strange angle which throws off what you think is level...

I used to have this problem much more often than i do now, but using a digital camera with a digital level, has helped me a bunch... especially when looking up or down, you tend to compensate for wide angle distortion or what not by turning the camera slightly... It 'looks' right at the moment, but when you print full frame, which might be your intention, it's quite upsetting to realize you weren't holding the camera level to the horizon.

cropping does help, but it becomes annoying, when you end up cropping things at the edge of the frames that you intended to use compositionally.