Title says it all, should I have my text using a variable width font, or fixed width. I didn't know if it is easier to read if its aligned on a grid?
If you want to do it the old-fashioned way, then yes, you can use it in a grid. But if you want to use any kind of variable or half-width characters, like writing 5 instead of ５, then you'll need to get off the grid. The characters are all the same width anyway so if you're not using any non-Asian characters then it should align by itself. I don't think having it not on a grid would make any difference in readability.
As a demonstration:
The question of whether it should be in a grid is really up to you. If you're dealing with presenting data, like a statistics screen, then using a fixed width for all characters could make things easier. If you're just printing lines of text, on the other hand, it makes much less of a difference and you can choose freely, as you can find both ways in use.