The box to the left is a good example. Notice that you don’t see any cracks or flaws revealing the inside of the miter. Even with working as perfectly as you can though, sometimes the miter joint has a crack somewhere. That is typical because very, VERY few pieces of lumber are going to be perfect. And even if it was perfect when you first got the wood stock, once you put it in your shop for a few days it may adapt to the surrounding area and change.
Really though, it doesn’t matter. There are a few tricks to getting the miter to look perfect once you have glued it all together. Also, Izzy Swan shows us here a cool trick to pulling your miters together without even using clamps. So check it out!