Going to buck the trend a bit here; going deep isn't always your best move. Many times you won't be able to reach the bottom due to hitting max dig depth, water, bisection of node by large rock, or even roof of burial chamber or troll cave. Even without those factors, your ore pieces per hour rate drops past the top few layers as you have to move more and more material to expose the next chunk of ore and add infrastructure, defenses, etc.
It really comes down to how much copper do you need and how many nodes are nearby. Given half a dozen nodes nearby and only wanting a moderate amount of copper; I find it faster to just mine the tops down a few layers at each node and move on.
All that said, getting at least one copper shower in your gaming time with Valheim is one of those game defining moments :)