I've done this a few times when I've had to abandon a session for the night, but I always make sure that the tape is away from the heads. (And I clean the heads at that point too)
Way back when, I was always told to keep floppy disks out of the drive when it was powered up or down, since there may be a brief surge on the drive head. While I don't know if this happens on tape machines, it's better than finding out the hard way.
I leave tape on sometimes. I usually loosen it so the tension arms relax. And don't forget to shut it off so it's not sitting a long while with the arms engaged and capstan spinning.