I would only do it if the I spoke to the owner of the property that it was on.

Example 1: Bike chained to fence at apartment complex for weeks. After talking to the owner/employees at the complex, if they give me the go ahead, I would feel comfortable freeing it.

Example 2: Bike chained up at Marta/Metra train station. I would never liberate this because it would be more or less public property. Street signs and sidewalks are the same problem. Too risky.