How Do Carbon Spots Affect Grading For Copper Coins?

I recently sent a Brit Indian copper coin into PCGS for grading and it was a "Shoe In" for a MS65 grade But it had some carbon spotting.
It was absolutely NO surprise to Me that it came back with a MS64RD grade.
So my short answer is Yes carbon spots Do affect the grade given

Not only does it affect the grade, especially with NGC, if the spotting is significant it might get you an environmental damage, meaning you spent $23 or so for a coin worth one cent.

To "carbon spots" are just like a scratch, maybe even worse. They lower the grade at least a point if they are not too bad. They can hurt you a lot more if they are big, or there are too many of them. As moxking said, you can end up with cent in a slab that worth a cent or whatever the going price is for junk coins in a slab is these days.

The worst case for carbon spots was a 1963 cent that was in a PCGS PR-70 Ultra Cameo holder that had a number of spots that had formed on the piece in the holder. There is no way PCGS would have graded the coin at the level if the spots had been on the piece before it went into the holder. The coin sold for over $40,000 in a FUN Heritage auction. If the coin had been cracked out it was worth about $10. A couple of years later the same coin cropped up in another FUN Heritage auction. Once more the price went to 5 figures. Why? Registry points! It was "the finest known." Collectors were making cracks about the coin on PCGS blog. Finally PCGS bought the coin to avoid further embarrassment.