It seems as though every comic collector has his/her own acceptable grade guidelines. As much as I would love to only have uber-high grade gems, if that was the case, I wouldn't have very many older comics.
So what are my grading guidelines?
I go for eye appeal and strive to get clean, structurally sound, well-cut books with no major ink-breaking creases or pieces missing. Even though some of my more expensive books may be "technical 4.0s" due to hidden defects or an accumulation of small flaws, the book may actually look much better than the technical grade. The minimum grades I shoot for are:
Gold - VG 4.0
Silver Keys - VG+ 4.5
Early Silver - Fine 6.0
Later Silver - F/VF 7.0
Bronze Keys - VF- 7.5
Bronze - VF/NM 9.0
Modern - NM 9.4
I try not to obsess over condition and avoid the "must upgrade" mentality. I am not always successful.