Matt Black v Photo Black Ink

Most of my colour printing is done on photo matt papers but I do occasionally use glossy papers too.  My Epson printer utilises two full black inks.  One for matt papers called Matt Black (MK) and one for glossy papers called Photo Black (PK).  On my previous Epson printer (R2400) if I wanted to switch my media from matt to glossy paper I had to unplug the matt black ink and plug in the photo black.  the printer would then perform a head clean to remove any residual ink before printing.  My new Epson R3000 both blacks are installed but switching between them involves the printer flushing out the old ink from the tubes so there is no cross contamination.  Either way, it wastes ink so you don't want to be doing it too often or just for the sake of a one off print.

So what happens if you don't switch?
Well if you are using Epson ink with Epson paper then you probably haven't gone to the trouble to get any custom profiles created.  In which case you are stuck with switching inks as you cannot select any of the glossy paper profiles if you have matt black ink installed and vice a versa.
If you are using quality third party paper the manufacturers will supply generic profiles for that paper for your printer.  They will be on the paper manufacturers web site somewhere usually called ICC profiles.  You will need to select the paper & your model of printer and probably either MK or PK ink.  The profile will then be made available for you to download and install.  This is not a custom profile but normally these will be close enough.  For custom profiles you normally have to print of a set of colour patches which you send away to be read by a spectrometer and the resulting profile will be e-mailed back to you.  Once you have your profiles you can select them no matter which type of black ink you have installed allowing you to print on either matt or glossy papers with either matt black or photo black ink.

Matt Paper with Photo Black Ink
If you print on mat papers with photo black ink the results can look a bit flat.  The high absorbency of the paper means the black ink soaks in more and looks less black, more very dark grey.  As a result there can appear to be a lack of contrast in your images. If you are going to regularly use PK ink on matt paper I would recommend getting some custom profiles made up for each matt paper using PK ink and if you make sure you use them each time it will largely compensate for this problem.

Glossy Paper with Matt Black Ink
Printing on glossy paper with matt black ink is not generally recommended by ink manufacturers.  The ink looks dull (matt) on the surface while all the coloured inks will retain their gloss.  You will notice this when you tilt the image to reflect the light.  The ink will also not penetrate the paper so the black areas can be rubbed off.  Having said that, if you look at the image flat on you will not see any difference in colour or contrast from a glossy print printed with photo black ink and you can solve the rubbing off problem by spraying the image with fixative spray.  You can even get fixatives with a glossy finish so eliminating the matt texture of the black areas.

Verdict.
Swap the ink over if:

• you are going to be printing an image for sale or exhibition
• you are going to be printing a large volume so making it worth changing.

Leave the ink as it is if:

• you are just printing the off image for yourself 
• the images are not of high importance.

Ideally, you need to use the right ink for the right media if you want the absolute best results but we don't always need that level of perfection, do we?