The answers above are good, and I agree.

But, I'm reminded of a quote by Billy Carter, infamous brother of ex-President Jimmy Carter.
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In an interview, it was mentioned that he was a peanut farmer, and the question was...
"In these times of health conscious people, tell me Billy, are peanuts fattening?"

Billy's reply was "Welp... It depends on how many ya eat."
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I think each person has to decide what's "good enough" for building. "Perfect" is probably the best answer, but most of us stop somewhat short of that, depending on "things". Personally, I think a flat and level workbench is the best tool we can have, and hope for the day when I can afford the stainless steel, or precision-ground granite, or whatever. In the meantime, it's (for me) a lot of measuring with incidence meters, digital calipers, etc., and striving for the best.