My iPF6100 is brand new, but I had an iPF5000 for years before it, and it drank the inks with "photo" in their names (plus the yellow) noticeably more quickly than the others. I print mainly on photo black papers, so the matte black rarely gets used.
Most used:
Photo Gray
Photo Magenta
Photo Cyan
Yellow
Intermediate:
Gray (one cartridge died with ink in it, so I don't have a clear sense of how much it would have been used)
Red
Green
Blue
Least used
Photo Black
Matte Black
Cyan
Magenta
-Dan