While I've seen more than a few proof 66s with a few hairlines (those are NOT staple scratches), personally, I prefer my 66s without them.
Grading scale for Proofs PF-60 Well it's a proof and that's about all we can say for it' PF-61 has a few less marks than a 60. PF-62 has a few less marks than a 61. PF-63 has a few less marks than a 62. PF-64 has a few less marks than a 63. actually starting to look decent PF-65 has a few less marks than a 64. looks fairly good PF-66 has a few less marks than a 65. PF-67 has a few less marks than a 66. This is a nice coin PF-68 has a few less marks than a 67. PF-69 has only a couple marks PF-70 has no marks.
Not sure if I should admit this - but I got it too. I think the best thing to do is to present the image of the coin as a whole. Same with RLM's guess the grades - you have to remember you are seeing a coin larger than 5x and you need to take that into account. Sometimes that is tough.