Henry,
The operation of the XFER pumps are mandatory as laid down in the manufacturers flight manual, the procedures in this manual may NOT be altered by local operators. The fact that some pilots apply common sense to this requirement is outside of the regulations. I can't be clearer than that. The drill is, Amber Caution and master caution illuminate, Cancel master caution, check CAD, Check fuel in main tank, Check Switch to on, Check CCB in. If all are okay, switch pump off. Pilots will know if the aircraft attitude has caused the caution to illuminate, and this is where they (some) apply common sense deviate from the regulation.
As to the times at which the amber aft or fwd pump cautions appear, it has been explained at length earlier in this thread, and will depend on the amount of fuel in the main tank at the start of the flight and the aircraft CofG and attitude. We're certainly not talking about the whole flight here, perhaps typically on the G-SPAO last flight the last 30 minutes (if it had landed at base with the MLA remaining in the supply tanks).