Is not using the full screen an issue?

This is my first projector/screen (I'm a noob) My dilemma is as follows-
My room has a 7' ceiling, my back row of seats in on a 8" riser, I have just purchased a Panny 8000 with a 106" motorized screen. Due to the height of the back row and the low level ceiling i will not be able to fully drop the screen to reveal the black border at the top of the screen and getting the screen to display a true 16:9 ratio (It will almost seem like you have to look downward at the bottom of the screen if you are in the back row). Will I have issues trying to get a proper picture displayed?(not stretched) Will there be contrast issues with no border on the top? Is there any other foreseeable issues I haven't thought about? I am trying to figure this out before actually hanging the screen, I am leaning towards returning it and going with a 100" motorized. I'm just not sure if there would be any point in getting a 100" if I could do everything on a 106" that I could with the 100". I got the 106" motorized screen for $140, hence the hesitation on the return..

BTW- I have a 65" TV on the wall so fixed mount is out of the question.

Thanks in advance!

Thanks airscapes, helped me see this for what it really is... The back rows problem:D Mind made up, 106 it is!