An autopilot in icing can also mask trim changes, including aileron forces. (In the Roselawn ATR crash they had "control snatch" in roll due to disturbed flow over the ailerons.)
If you are at a stable indicated airspeed and suddenly have to start trimming, and you're in icing conditions, I'd say the time to act is upon you.
If the autopilot is on, the worst thing possible will happen: it will trim and trim until it runs out of authority, then disconnect and say, "you've got it!"