One difficulty encountered when transferring cassette tapes to a digital medium is the issue of proper azimuth alignment. Essentially, this boils done to differences in the direction the play head in the machine is aligned with when tape is passing over it. The goal is to have the same alignment as the original machine which made the recording. This will not be possible in 99% of cases, unless you are using the same deck.
Here is one excellent article and a fine video on this subject. Anyone who is very serious about helping to preserve recorded musical culture will hopefully be motivated to take a few minutes to read a couple of other articles as well.