tape in deck when off????

Depends on the deck. Better decks disengage the pinch rollers when powered down. I can't see much harm in leaving the tape in for such a deck. Leaving the tape in a deck that doesn't disenage the pinch roller can cause the roller to deform from the constant pressure over time - not a good thing.

You actually use cassettes still?....just kidding

Take the tape out. Leaving it in the deck is bad for both the tape and the deck. Some of the issues are mentioned above. Your tapes will remain problem-free much longer if they are rewound to the end and kept in their cases when not in use. I also would recommend to take the tape out “before” powering down the deck.

Best regards,