They only affect brightness level not necessarily adversely but changing G-CUT will move the brightness level of the low-end of the greyscale.
Yes definitely start at YOUR original values and apply the new offsets. These new offsets are for each of the three color temps, I believe you press the 7 button on the remote to switch between color temps in the service menu.
My other settings using these offsets are as follows:
Video NR Off
MPEG NR Off
Black Level Light
HD-Size 2 for blu-ray and 1 for everything else.