If you're going for the No Assist medal on every puzzle, it's useful to know what does and does not count.

You can use Hint Number Auto Check (from settings menu) and still get No Assist medal.

You can't use Check Mistakes (from the pause menu).
You can't use Hint Roulette (from settings menu).
You can't use Autocorrect Mistakes (from settings menu).

You can't use Highlight Incorrect Lines (from settings menu).
Actually, technically you can, but only if the red highlight never lights up, so it's really not useful. May as well keep it off so that you don't get penalized for a misclick.

If you make a mistake, then Rewind to before it happened, you still receive the No Assist medal. Unless you're using Highlight Incorrect Lines and they light up red. You can't rewind that.