For now I only have a cheap 12" Stanley combo square that I've fettled as best I can to be knife-cut accurate using the flip-method.
I might get a proper Starrett or similar brand one day, but so long as it's accurate enough for what I'm doing... both what work and how accurately I can work - No point in quarter-thou accuracy over 20 yards, if I can't physically cut to a straight line in the first place!!
But really, so long as you can square the square up (which you should be able to do anyway, in case you drop it or something, IMO), I don't think it matters that much...