mixing ink

If you print photos (on photo paper) then you use the dye-based black (denoted K often times I gathered) instead of the pigment-based black (Bk). Now when does this "photo black" actually get used? Is it only for the parts of the photo where it is completely black ("true black")? Or does it also...

The strategy that I'm most comfortable with, as explained by @jtoolman, is to swap out all ink cartridges once just one becomes low. This reduces the number of cleaning cycles run by the printer whenever even just one ink cartridge is replaced. For purposes of this discussion, "first carts"...