I have a steel 24" straight edge that has a slight bow to it, and it's bugged me for 25 years. This is the type that's about 1/8" thick and one edge is beveled. I can get a "machinist's" version of the same thing, accurate to .001 over its length, from McMaster for about $115 or $64 without the bevel. I know that's overkill for woodworking, but reference tools should be dead on IMO.
A carpenter's square doesn't stand on edge, and plastic rules are too easily dinged. What do you guys use?