Ice will destort and destroy the lift performance of your wing. Lots of CRM case studies with regards to accidents/incidents resulting from departures with ice adhering to the wing.
Freezing rain with signs of ice accretion before departure? Then de-ice and apply anti-ice measures if applicable to your aircraft type.
During flight? Anticipate (visible moisture below 10c) and deploy anti-ice and de-ice measures as are available to you. Not possible? Then climb or descend or return the way you came (if its an option) until clear of the condition. NASA did some research and found that ice appears in layers of 2000-3000'....apparantly. Not sure how they worked that out...
Or better still, listen to the man from the CAA:
See ice? Dont think twice! De-ice! 