I am digitising cassette recordings made on normal-speed cassette players and also on a variety of double-speed Tascam units. Speed variation is normally not a problem, i.e not noticeable, but there are some cassettes where the playback speed is obviously not right, caused either by a change in deck speed over the years, or playing back on a different deck. I digitised anyway, thinking that I would change the speed in Audacity. However, I’ve had second thoughts. The Tascam recorders have the ability to vary speed. Is there likely to be any difference in sound quality between playing the recordings at correct speed while digitising (this may introduce problems with Dolby and dBx encoding, or it may not), and playing back at whatever speed and altering the speed in Audacity?
Change Speed is the least damaging of the time stretching effects. I’d take a representative cassette and re-digitize using the deck with variable speed, then do change speed on the first copy, then compare them. If one sounds better, go with that method. My understanding is that Dolby B is pretty forgiving (it had to be to be used on cassette decks) so minor speed variations on playback would not, I think, be an issue.