According to Bernie Grundman---who has been mastering since the 1960’s---analogue masters tapes are relatively hearty, and suffer no degradation from sitting on a shelf unplayed (assuming the shelf is in a climate controlled environment, which is generally the case). What DOES degrade tapes is being played; the more they are played, the more oxide particles are "shed" from the backing polyester film onto which the oxide is applied. Tapes which have been store unused for fifty years have been found to have startling sound quality, assuming the recording sounded excellent to begin with. To paraphrase Mark Twain’s joke about reports of his death, reports of tape degradation by the mere passage of time are greatly exaggerated.
The usual practice is to make a "production master" tape from the original 2-channel (if stereo) final mix tape, the original master tape then being put away. Safety copy is another term used in place of production master. That production master tape is then used to cut the lacquer from which the metal pressing "plates" are made. The plates are installed in the LP pressing machine, and voila, you have a vinyl LP. There is another step involving father and mother transfers, but this is already complicated enough. 
For the Analogue Production mastering and pressing of Miles Davis’ Kind Of Blue, however, Chad Kassem was able to get the original multitrack tape (only three tracks. Remember the album was recorded in 1957), which had not been played since the original production master copies were made at the time of the album’s original release. Grundman said the tapes (two of them) were immaculate.
Rather than making a new stereo production master copy from the 3-track master., Bernie mixed the three channels and sent the resulting 2-track mix straight into the mastering console, thereby eliminating one stage of analogue tape copy degradation. THAT degradation is real.
But there’s even more to the story. While playing the two master tapes---one made each day of the two day sessions for the album, and each on a different 3-track machine---Grundman discovered that one of the tapes was playing back at the wrong speed, thereby changing the pitch of the instruments and the "feel" of the songs and musicianship. What had happened is that during one of the two sessions, the recorded used that day was running either slightly too fast or too slow (I don’t remember which). Hence since the tape was then played back on a playback deck that was running at perfect speed when the lacquer for that LP side was cut (the two days of recording were placed on the two opposite sides of the LP), that side of the LP is out of tune and time. And that was true of every version of Kind Of Blue ever made up to that point in time (1997)!
Naturally Kassem had Grundman adjust the speed of the playback deck to get the music on that side of the LP back to the speed it had been played at in the studio. Once again, Bernie Grundman to the rescue! This is just one more example of why Analogue Production LP’s are as good as they are. And we haven’t talked about the sound quality of the mastering itself, and the quality of the QRP LP pressings.
