Blueprinting... Educate the uneducated
I'm assuming it's good to start with a "blue printed engine" though when building high performance engines though? Or even building a strong reliable engine?
For a performance engine, it's considered a must. Most builds focus on more power, and the best place to start is often compression. Higher compression leads to tighter clearances (piston and heads coming within .035 of each other in some cases, versus within .125 or more in a factory motor) tighter clearances mean you can't deviate part-to-part as much. A connecting rod being .010 longer than another could be catastrophic. This is just one example. Piston to bore clearances, ring end gaps, valve heights, spring heights, etc... it all matter and has to be exact.
Then there's tolerance 'stack up', which is the combination of tolerances which compound one another. Think of the piston - it's attached to a pin, attached to a rod, attached to the crank, in a bearing, in a block. Even if every tolerance was kept to +-.002 (total width equal to the thickness of paper), that adds up to +-.012, or .024 total tolerance. That's 6 sheets of paper...
Just for reference, when I measured my piston-to-deck height on my last shortblock, the variance was less than .002 - with all of those tolerances 'stacked'. Just imagine having a shortblock where one piston is sticking out .012 and another is sunk .012. Hard to relate if you haven't 'seen' what .012 is, or what a precision build results in, but it would be pretty bad to have such a result.