mitering certainly has a bigger effect on alignment of tig or fillet brazed frames, but is also important for lugged frames. I'm fixing a frame right now that had some mitering issues and also had very thinned out lugs, so when the builder also made the vent holes way too large, there wasn't enough margin to keep things from failing in fatigue. Badly mitered frames were a thing you can find on production bikes, not reputable builders. Some of those badly mitered production bikes aren't really thought of as production bikes by many, but those companies are pretty much gone now.