Digital BW, The Print

Although my 1400 can accommodate and print with six different tones of ink, in my black-and-white printing (the only kind I do) I utilize and print with only three dilutions of black, using QTR curves.  I also employ a CIS system.  For the three ink channels I do not employ (which presently contain gradually-depleting dilutions of ink I do not use) is it safe to just let them eventually run dry, through evaporation or the occasional nozzle cleaning, or should I substitute the ink in their respective three CIS compartments with some benign replacement solution? I mix my own dilutions of black using Paul Roark's base solution formula. If it is inadvisable to let the channels run dry, could I use the base solution as a replacement, or should I use some cleaning solution?   

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