In a B737NG, assuming you have fuel in all tanks, Wings and Center, and that all pump switches are ON, will the fuel from the CTR tank be the first to be pumped, despite all Fuel pump switches being in the ON position? Will this happen even if the CTR fuel pumps are OFF and only the Wing Fuel pumps are ON? And why are there so many warning in the FCOM about pumping the CTR tanks and warnings about not doing so in climb or descent if under 1000 lbs of fuel in the tank? I have already read the "Why are fuel tanks in the wings filled first, and why are they used last?" post and answers.