granite squares get checked like anything else, 36" granite squares i use are within .0001 per 36"
some are marked on edge per inch with a type of ruler sticker. so you can lookup error say at 10" mark
might say its -5 or -5 millionths there at 10" mark
the original manufacturer can probably "fix" any worn square if really worried about the error in millionths
No, the sides will not be square unless you get super lucky. One can clamp a precision square onto the top of the plate for a somewhat true 90 reference.
The sides also will not be square to the top no matter how nice they may look.
Well that seems like pocket change... and beware a 2 side and 6 side much different when using even though they look the same.
Set a two side on it's back and it may no longer be such a good 90 if real fussy matters.