Should low branches be left on black pines growing in the ground to increase trunk diameter ?
Two things to note about this:
The trunk will thicken below the point where the branch emerges so the branch should be high enough to cause thickening where desired. The exact location is determined by the size and style of the trunk desired.
If keeping low branches to thicken the lower trunk, try to place them in the back so scars aren't visible from the front.