We do know that Michelangelo copied from the statues in the Medici palaces, etc.. This very practice, followed by Raphael's refining of it, is really the genesis of modern academic practice. [...]
Nearly all the great artists one can think of earned their ability to deviate from "reality" by first understanding what things look like. The way to do that is to draw as accurately as your understanding allows. This takes practice, and if you're lucky, a good teacher by your side to speed things up. Over the centuries there were schools of thought or teachers with approaches that didn't expect super-accurate drawing. However, I think that by the time the 19th c. academies rolled around we had accumulated quite a bit of drawing knowledge and it would have been fairly well agreed upon that accurate mimesis is the best and fastest way to LEARN to draw well, whether or not your finished pieces are super-accurate or not. Again, it's a matter of mastering something before you can properly deviate from it knowledgably and artistically.
Juan
