I have many and use different ones for different things. Whenever I can, I prefer to use my 4"x14" rectangle. It's easiest to handle. If I need a wider one, I've got 6"x 18", 6"x24". If I'm cutting strips, I LOVE the June Tailor slotted ruler! For squaring up, I've got various sized squares. I'm all for quick & easy. If it's easier to buy a certain size to square up a lot of blocks, that's what I do.
Always wait for a sale of course!