Ya, I always expose the entire vein of ANY ore before harvesting...and just give yourself a way out of the pit. I find that this keeps the annoyance factor of spawns interrupting work to a minimum. Plus, I just find exposing the entire node makes mining easier, if you dig under far enough, by striking and destroying the most bottom part of the node connecting it to 'foundation' it will instantly splinter the entire node....unless that has changed?