We have several different printers in-house (Dye-sub, solvent, UV flatbed, pigment inkjet), so my 64S may go several days without being used. Is the "Fill Cap" mode the best way for it to sit for a few days at a time?
Everything depends on state of cappers and for how long you will have printer shut down!
In short: - if cappers are in good condition, when closed, they make good seal of head - than "fill cap" performs like it is supposed to - cap is filled with ink and heads are soaked in it! - if cappers are in bad condition and you don't have good seal of the head, results with heads not soaked in ink - result of that is drying of ink on heads...
So if you are shuting machine for 2-3 days this doesn't matter (ofcourse if we are talking about normal working environment), but if you want to leave printer for more days, than you should be sure in state of your cappers!