Hi Steve. If IR is not blocked, you'll notice on bright stars it wil be harder to get accurate focus because your camera is letting in IR spectrum. talking about camera body at prime. This is the case also with my DSI when i removed the IR blocking filter, the image will be more red than without and stars will become larger.
corrective action i have taken with my camera is to get myself a UV/IR filter (for astro use) and for terrestial use, getting myself the IDAS daylight front filter
The IDAS daylight front filter is very very close to the original canon filter.