Thanks for the responses so far. I would like further enlightenment, though it would seem (upon further reading) that in most aircraft the crossfeed remains closed unless an imbalance is noticed (but not a fuel leak). I think what has confused me was the 747. Perhaps it is a different breed from the others but I can remember many years ago, when riding up front with my father, the crossfeed was open. The EICAS showed a yellow "XFEED" warning until the crossfeed was open.
Unfortunately, my father is no longer around so I can't ask him. Maybe just one needs to be open (as Earl mentioned for the classic). I was only an ignorant teenager during that flight but was very interested in all aspects and all the details as I am today.
I appreciate your answers.
Cheers!