I don't know for sure because I don't fly the 747, however the same thing is done in the 737 where the centre tank pumps are actually located in the main wing tanks. It isn't written in the vol 2 so you don't need to know it, but I suspect it is as you say for cooling. The centre tank fuel is used first and then the main wing tanks, so the centre pumps should always be cooled by surrounding fuel even when the centre tank is empty. The theory also being that the wing tanks should never get that empty
