No magnetic tape can be completely erased, even in a bulk erasing machine that you put the cassette into. (This is why data tapes that contain classified information cannot be declassified except by physically grinding them up). However, the residual signal is so faint that it cannot be heard except perhaps during silent periods between selections. It is less noticable than groove to groove crosstalk in a LP.
The term "bleed through" generally means the transfer of signal from one part of the tape to another part when the tape is tightly wound and stored for a long time.
