Digital BW, The Print

I am setting up an Epson 7000 with Piezzotone selenium inks and see that the inkset 
contains 4 inks. What do I do with the two other channels? Surely something has to be in 
there to keep the heads from drying out. Do I order carts that are clear medium to put in 
there? In the future I would like to try using a sepia cart in one of them if possible.

Also, if anyone has any experience or suggestions about using QTR with this set up that 
would be very helpful. I know I need to have profiles made for that rip procedure.

Thanks,

John