The purpose of up or down thrust is to minimize pitch attitude changes with throttle variation. If the thrust line falls above the center of drag of the aircraft, then up thrust is required to compensate for the thrust-drag couple which tends to pitch the nose down. If the thrust line falls below the center of drag then down thrust is required. If the thrust line is aimed through the center of drag then the moment arm of the thrust-drag couple is zero and the pitch attitude is unaffected.

A low aspect ratio delta flies with a lot of induced drag caused by the vortices at the wing tips. Induced drag tends to dominate the drag budget of the whole aircraft. Running the thrust line through the wing center is a reasonable starting point for adjusting the thrust line on a delta which has no dihedral or anhedral.