This is pretty much the same position that ASME IX takes in CVN testing. A vertical up weld is assumed to have the slowest travel speed, thus producing the highest heat input. Since notch toughness is related to heat input, the idea is that if you get acceptable results in 3G, you will get acceptable results in other positions. Note that this does not set aside the need to qualify other positions by PQR, at best it simply means you only have to take charpies from the 3G plate.