Agustin, where you ask about "wasting ink in any way," you may be thinking about how the P800 handles switching from Photo Black to Matte Black for different media, where some ink is consumed that is not actually used to make a print. In my own work I switch whenever I need to, and in any case do the switch weekly anyway in order to keep the mechanism for switching between PK and MK from becoming "stuck," which I have been told can happen.
I print a color gamut test chart virtually every day, just to keep everything flowing nicely (sorry, this is not the technical term; perhaps others will care to comment on this), and have never had reason to think my BW printing was affected by this. The Advanced B&W mode uses a small amount of color.