I use a quilter's plastic see-through ruler, with a blade on a cutting mat. It gives a nice crisp, straight edge, but even so, it's sometimes a mm or so off-kilter. It's barely noticable.
For doing watercolours that have a margin around them, I use a template that I cut that I know is exact, and pencil around it with a very fine point.
I also cut a "window" from cardboard that I use to check composition and colour balance when need to hide the margins, that sometimes look messy when I drag paint off into them.
/Karen