frankly it just depends whether you are looking for phrases, sounds, or other elements.

although its easy to see the value of rare records for sampling grooves, you should certainly realize that on more famous albums, etc, the bigger the budget, the better the engineers often would have been. these are good for single notes, drum sounds, etc. Unless you go crazy and sample something ABSOLUTELY RECOGNIZIBLE (ie the stomp/clap loop in queen's we will rock you) its unlikely that you'll have a prob if you're using these for source material.

-lodger