To all,
In the NG, fuel is always being transferred. from centre to wings.
As pilots, you would know that you aren't generally allowed to transfer (with your boost pumps) anything under roughly 4 -500kg.
In the Ng, your scavenge is always on, so on a long flight, you will notice the centre decrease, but maybe not on a shorter flight.
In the older 737, there was a 20 min scavenge once centre pumps turned off, and as long as your wing tank pump(namely left forward I think) was on, but now it is always on.
hope this is totally correct, but feel free to correct me if you want, as this is from memory.