Well, when I refer to "LR balance" it's similar to "panning" but not exactly the same. Yes, it's about positioning the instruments so we have a better stereo effect. But for "LR balance", it's more like "balance the loudness of the left and right channel so they are the same".It's nothing with the stereo field.
For the loudness, I still prefer to make a final check when mastering. During the mixing step, we could try to make them the same but there is no guarantee. We can always make some mistakes and leave one track a bit louder than the other. Setting all 6 tracks in a row and checking them one by one is a lot easier than setting the loudness in each track.