Here's a good rule of thumb - ANY surface that you can place a mirror flat against, and see either speaker from the mix position, if the distance from the speaker to that surface and back to your head is not AT LEAST 20 feet or more LONGER than the DIRECT distance from speaker to your head, you will lose clarity of stereo image UNLESS you place some type of absorbent material on the offending surface, or with longer distances diffusion.
This is true of walls, ceilings, equipment racks, large monitors, (which, BTW, are also usually excellent resonant cavities, some with neat vibrating metal goodies inside) dogs (especially if their coat is verrry shiny :=)
Whether you notice this phenomenon or not, will depend on your ears, speakers, amp, program material, wall treatments in general, etc, but yes, the monitor CAN and WILL make some difference in sound quality if it's in the sound field.
I use a 21" Iiyama monitor, and had to take the back off and put some foam deadeners on a couple of the metal chassis covers to silence some rattles, and some 2" Sonex foam around the cabinet to get rid of the early reflections off the cabinet sides.
You can also get lots of early reflections off the surface of the mixing console, which is why some people tilt the surface of the desk down in front so that these early reflections are directed below the ears and to the back wall... Steve
