If these cassettes were recorded in mono or have minimal stereo seperation, you will want to consider using only one side (right or left) of the cassette deck output to avoid phasing errors. Phasing is that "swishing" sound that you sometimes hear, which is actually phase cancellation. Most cassette decks are/were out of alignment, so cassettes recorded on one may play back out of phase on another. Listen to these tapes in mono through your stereo and you may hear the swishing. If they play back fine, you can go ahead and use the stereo output. Otherwise you may want to choose the right or left side and run it into a y-cord and then into your iMic, if you are using one. Some macs can take a direct line level input. I also am a fan of Felt Tip Software's Sound Studio for recording. It's expensive to register, but can be used as freeware.
Val