There is a setting to disable the auto nozzle check, which you may already have turned off. I find that when the printer sits for any more than 2 days, the first thing it wants to do is a quick clean or nozzle check. As long as I print out a nozzle check or do a small print every second day, then I can circumvent this procedure, but if I wait for 3 days, bam, I've missed the window of opportunity.
I have a 6400 now, and it has great account software so I can monitor this better going forward as it gives a nice total ink count value, but I use it almost every day so I'm not sure if I will come up with numbers for how much ink is wasted after non use for 3 days and how much ink is actually used between the last time it was used, and after the auto nozzle check and clean. I imagine this clean cycle isn't the same as a clean cycle A which wastes about 10ml of ink. It probably just spits out a bit of ink, but I have no numbers. It probably though is much safer to just print a nozzle check every two days or at least an 8x10 print which is only 1 ml of ink because I do fear those cleans actually waste more ink than what you would use yourself to print a bit every two days.