It isn't as important as you might think, but a good rule-of-thumb is 2%.
Dave Crobin tells a story in one of his books of swaging a bunch of bullets, then sorting them all by weight, setting the ones that were closest aside for later loading. He woke up the next morning, loaded the bullets and went off to his match where he had the best days shooting of his life. He decided that he'd found the magic solution to accuracy.
When he got home he went back to his swaging bench, and something looked amiss. He weighed the bullets he'd "culled" and found that he'd mistakenly loaded the wrong bullets, and the uniform ones were left at home. He'd had his best day of shooting using bullets with the greatest range in weights.
He concluded that there were other factors that were much more important to accuracy than uniform weights - one of them is confidence in what you're shooting.