Thanks for that clarification Jon. It's consistent with everything I've read about the MK/PK switch in these printers. It was just that your comment read as being specific to the R3000.
If you plan to "NEVER EVER do the black ink change", then I'd have thought that flush in the unused K channel would be the best option as it would most likely give you the best chance of success should you ever change your mind. This is what I'd do if I had a 3880 running P2.
If it uses the suction at the capping station mostly, I assume that other inks will be consumed during the process.
It's my understanding that in these ink-lines-and-dampers printers, the MK/PK switch only consumes the black ink being switched to, and none of the other eight channels. So in that respect it's different to desktop printers like the R2880 and earlier, and also different to switching black cartridges in your K7 R2000. I can't quick find official figures for the R3000 ink wastage, but Keith Cooper in his review quotes:
[I]Black ink conversion times
. Matte to Photo Black approx. 3 min. 30 sec
. Photo to Matte Black approx. 2 min. sec
Ink used during conversion
. Matte to Photo Black approx. 3 ml
. Photo to Matte Black approx. 1 ml[/I]