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Beginning in 1969 the US Mint began to improve their proof product for collectors.  There were some "misses" along the way, however; today it can be safely stated the "common" grade for any proof coin taken from a proof set of recent vintage (1990 and later) should be at least PF68, cameo.  I say cameo as the mint process of producing coins for proof sets results in all more recent proof coins having a cameo surface.  The lack of a cameo surface would be a reason to reject the recent date proof set or coin.

This is important as many sellers will imply they are offering a PF69 with either heavy or deep or ultra cameo at a premium.  These are the "usual" grade/cameo condition.  Additionally, you should use care about the 3rd party grading service used.  Many do not seem to have much quality control.  I have personally purchased and display a PF70 Roosevelt dime from a service below what is considered the top tier companies that has an actual scratch, which I assume came from a poorly removed staple, on the reverse.  Obviously, a coin with any problem that followed release from the mint should be noted as some grade and that it is an IMPAIRED proof.

Most modern proof coins are very reasonable - so using a better (or the best) third party service is not any major financial problem. 

Good luck and please - BE CAREFUL!