I avoid them, personally. I had a 60-degree triangle template (not a ruler, a template) and I ended up shaving one of the sides down with my rotary blade by accident, and not noticing until I'd cut a bunch of triangles that were about 1/8th inch too narrow on one side. Naturally out of fabric I was already short on.
So I cannot be trusted with them, and must use a thick ruler that I can't slice into, because I get a little wild with the rotary blade, apparently!