In a reflowable format like epub or azw3 you should never be aligning things based on precise measurements like grid lines. Chances are your alignment will be completely screwed up on different devices.
But if you want a grid, you can create one yourself using CSS and a simple image.
http://stackoverflow.com/questions/3...-with-just-css (see the second answer, with the image)
Just use that css for body. And comment it out when you are done, to remove the grid.