For bookbinding, the rulers are multi-purpose tools. You will use it for precise measurement, cutting, as a spacer, as a weight, and for many other jobs.
If you are a hobby-bookbinder and don’t have an industrial paper guillotine, you need to use the ruler (and a blade) for cutting. Smaller paper trimmers could be helpful but will only partially replace the ruler.
Steel and Aluminium Rulers
I have tried cutting rulers made for crafting purposes. They are made of thicker aluminum or acrylic, with steel edge reinforcement to help to lead the blade and protect the ruler. However, I still feel that a sturdy steel ruler is more suitable.
When choosing a ruler, all dimensions are equally important. The length is obvious: for a long cut, you need a long ruler, but it’s harder to handle, especially if you have a smaller work surface. You can buy common sizes like 150 / 300 / 600 / 1000 / 1500 mm.
The usefulness of the width could be less obvious, but steel rulers are great for using as a spacer or when cutting turn-ins in box-making. Also, since it is steel, it’s sturdy and easy to clean.
We can utilize the ruler’s edge to guide the cutting knife. The thicker the ruler, the easier to keep the knife at the proper angle. Also, it’s a bit safer since it’s harder for the blade to slip over the ruler and cause an injury.
The other benefit of a thicker ruler is that you can use that, combined with your square ruler, to cut perpendicular lines (for example, slicing a board with many parallel cuts without an additional measurement).
The heavier the ruler, the better it keeps the paper in place when cutting. So cutting a large sheet is much easier with a heavy ruler. But watch out. A meter-long thick ruler could also make it harder to move around.
Heavier and thicker rulers sometimes have a handle to make lifting and positioning easier.
Finish / coating
If the ruler’s surface is slippery, it is harder to keep it in place (especially when cutting a coated paper). Some rulers have a bit matt / rough surface. For slippery rulers, I apply masking tape to the back.
Some of my Shinwa squares have a white painted coating. That makes them ‘snap’ to the surface and easy to read, but it’s also easy to damage the coating layer with a sharp blade.
Some steel rulers are super flexible (and thin). Sometimes it’s an advantage. For example, it’s good to have a lightweight, flexible ruler for measuring, and also work for shorter, more precise cuts.
Scale and markings
I prefer to use millimeter markings, but I also have some inch rulers to make it easier to work with projects without constant conversion.
I prefer scales with deep-etched markings. That makes the marks more readable and more resistant to scratches, glues, and other damages.
Many millimeter rulers have half-millimeter graduation on one edge. I’m not fond of that, it’s much harder to read that kind of marking, and when I need half-millimeter precision, I can do that just by eye.
You can find many designs: graduation starts right from the edge of the ruler, or not. Sometimes you have the scale reading left to right and right to left on different edges, and so on. Again, it’s better to research before choosing the right tool for the job.
Good quality brands are Shinwa (Japan) (sometimes under different brand names, like Kokuyo), Vogel (Germany), Starret (US). Rumold (Germany) also offers steel rulers and precision rulers.
I keep the long rulers hanging next to my workbench, and the smaller ones are lying around. You always need them in bookbinding. It is worth keeping rulers clean. Any glue residue can make your cut uneven or even cause an injury by diverting your cutting motion.
I have some Shinwa squares, and those are indispensable for cutting perpendicular.
I don’t use plastic rulers for cutting, but I do use them to make marks. I especially like the acrylic rulers, which makes drawing parallel lines so much easier. I also have some plastic set squares (or triangles).
(The ruler at the bottom with the groove is a unique Japanese painter ruler for painting straight lines. You grab the rode with the brush, and the rod rides in the grooved ruler.)
I always keep some short 150mm Shinwa rulers around for smaller objects. It’s good to have a 600mm ruler, and sometimes also a 1000m ruler, plus a smaller set square (even a plastic one), and that’s it. But there is no such thing as ‘too many rulers.’
If you are looking for a good ruler, I recommend checking Dieter Schmid Feine Werkzeuge.