Have a look at: https://www.pcgs.com/eyeappeal/
Apparently, PCGS does take account of toning as part of their "eye appeal" assessment when assigning the grade. Supposedly, MS or PR 66 must have "above average" eye-appeal. High end coins for the grade, i.e. "plus" grades, cannot have negative or below average eye appeal for the grade.
Can't say I agree with them on some of the examples they gave. Personally, I find their two Morgan dollars with rainbow toning assessed with positive eye appeal positively ugly. The Peace Dollar with "below average" eye appeal is terrible but the reverse of the Morgan dollar with the lower category of "negative" eye appeal looks kind of OK to me.