As far as I'm aware it's a big no no from boeing but im ready to be proved wrong. Only time fuel should be left in the centre tanks is if you/ your aircraft doesn't have the fuel pump mod or approval to use it and you have to leave 500kgs in the tank to keep the pumps saturated.
On a slight side note does anyone happen to have a rough fuel pricing table. Many companies produce them giving an idea of where the fuel is cheap so you can decide to tank or not. My old company used to have one but my current company doesn't so i'd love to have a look at a more current one. Euro Ops.