I already replied in another topic but just in case you miss it, i'll repeat it here too.
The fuel cocks behind the seat select which feeder tank each engine is fed from (left/right tank or both).
The rest of the fuel controls in the switchboard (on the left side of the cockpit) are probably fuel pumps.
What really happened in the Ju88 is that the engines were fed only from two tanks. To get the fuel from the remaining tanks the pilot had to pump fuel from, for example, the auxiliary tank in the bomb bay to the feeder tanks.
In other words the fuel tanks are serially connected: tank 1->tank 2,etc->feeder tank->engine
The switchboard just allows the pilot to select which tank's contents go into the feeder tank.
That being said, i don't know exactly how it works or even if it's correctly modeled in the sim, so i turn everything on just in case. This might not be optimal in case of a fuel leak.
On the other hand, if the sim does model the fuel system correctly, then turning on only the two fuel cocks hidden behind the seat can give you surprises: you might have tons of fuel in the remaining tanks but if you don't pump them to the feeders, your engines will quit once the feeder tanks are dry.
I had once found a good resource about how it all really worked but i can't remember where i put it on my hard drive