Often the magazine dictates the maximum length. Many new rifles have shown up opening day and the magazine won’t feed because the cartridge length is too long, so make sure and cycle at least one full magazine before the season.
If the magazine doesn’t limit length, .020” is commonly picked because it’s close enough without usually causing issues. The danger of being too close, like just a few thou from touching, is slight differences in length might put some bullets into the lands and other not, which is never good.