How do you judge the quality of your haircut? How should you?
It's common to judge the quality of your haircut based on how it looks right after it's been cut, even though you know that it will be weeks before you return for another cut. Given that, it might make more sense to judge your haircut over the entire interval before the next visit.
Imagine one men's haircut that looks perfect today. After four weeks the hair has grown a half-inch, which is noticeably shaggy for the typical male professional hair length. On average, this guy's hair is a quarter-inch too long over a four week period.
For comparison, imagine if he had gotten the same style haircut but a quarter-inch too short on purpose. Although his hair is a little short on Day 1, by week four his hair has been only an eighth-inch away from ideal on average. Also, he still enjoyed one day of perfect length during the four week interval just like the earlier scenario had.
Does this extend to longer hairstyles? It does if the period between cuts is longer, which is often the case. If you wait four months between cuts, then that's two inches of growth. One inch off on average in the first scenario versus one quarter-inch off in the second.
An important assumption here is that your hairstyle looks equally good/bad whether it's a little too long or a little too short. If not, you can adjust the intentional extra chop to stay at optimal.