I'm trying to keep my everyday Traveler's Notebook kit compact, but the notebook itself isn't what takes up the space. It's all the little extras I pack just in case. I've started leaving the ruler and spare pen at home, though I'm not sure the ruler is gone for good.
For a quick straight line, the edge of a card or sticker backing is usually enough. I'd only put the ruler back in on days it's got a specific job.