I don't know if it actually "hurts" the machine. The dealers turn the demos on in the AM and leave them on ALL day and it doesn't seem to hurt them. I know my machine has lots more "ON" hours than "sewing" hours on her. I tend to turn her off when I finish sewing for that session. I don't turn her off when I'm just taking a break or ironing though.
Of course, I don't have any kidlets around.