The dealer I go to uses it as an approximation. Hes been dealing for over 30 years and can pinpoint a grade very well, but for the sake of his customers, he does the following;
XF = 40
ChXF = 45
AU = 50/53
CHAU = 55/58
Unc = 60/61
ChUnc = 61/62
BU = 62/63
ChBu = 63/64
Gem = 64/65
ChGem = 65+
He does this so people cant really argue grade with him. Which is not to say you can't challenge, but you better be ready to calmly and rationally defend your assessment, or he wont bother.
I've gotten a few coins at a reduction, not because I say "Hey, you over graded this!", but that "so I saw *this* and *this*, and if those weren't there I would completely agree with this price, can you work with me on this coin, I would be more comfortable at *$$$*."
It's all about the approach.
Seriously, if you are ever in the Lynnwood, Wa area, I highly recommend 'The Coin Market'. Tell them Ian sent you

. You wont regret it.