You gotta draw the line somewhere
So you're dealing with imagery that you place on a 2D surface and you have to determine if you have to custom cut a mat. What to use? A ruler. It's straight and it has the measurements right on it.
So you're dealing with imagery that you place on a 2D surface and you have to place a very straight edge on one of the items of your composition. What to use? A ruler. It focuses your pencil on the straight line truer than your hand alone does.
So why fight it? Use what you need.
I use metal rulers in the following lengths: 18", 24", and 36". The 24" and 36" rulers are 1/8" thick aluminum that is ideal for cutting straight cuts in heavier papers and mat board. The 18" is a thinner gauge ruler I use for measuring and drawing.
T-Squares and Triangles
If you are working on complex perspective work sometimes it pays to square up your paper on a drawing board, secure it, and use a T-square to draw your horizontals and a triangle to draw your verticals. T-Squares are available in wood, plastic, metal, or combinations of the three. Triangles come in a couple different angle sets: 30-60 and 45-90 degrees. The size and color choice is up to you, most are transparent plastic of varying grades.
The Compass, the Curve, and the Protractor
If you need a circle of a certain size and you don't have the right sized can to trace, a compass is a handy thing to have. A cheap (under $1) pencil-holder compass should serve most needs. French curves are used for drawing smoothly curved lines of various shapes. There's a protractor tacked to one of my bulletin boards to remind me which angle is which when I'm manipulating images in a paint program.
Recommended brands: Alvin, Fiskars, Grifhold, X-Acto, Staedtler, C-Thru, Gaebel, Discovery, Empire, Mars, Acme, Chartpak