In Chicago we must look to the Chicago Municipal Code which reads in relevant part:
9-52-070 Parking.
No person shall park a bicycle upon a street other than upon the roadway against the curb or upon the sidewalk against a rack, parking meter or sign pole to support the bicycle or against a building or at the curb in such manner as to afford the least obstruction to pedestrian traffic.
(Added Coun. J. 7-12-90, p. 18634; Amend Coun. J. 7-21-04, p. 28659, § 1)
Note that it is specifically legal to lock to a rack, parking meter or sign pole.
Having said that, don't call me when a property owner cuts your bike off the sign in his front yard. Common sense dictates that you must be considerate when parking on someone's property- especially in their grassy yard or parkway.
Also... Don't leave your bike in the same place for more than a week. If you do your bike may be deemed abandoned and it is subject to being removed.
See Chicago Muni Code: Section 9-52-071, regarding Abandoned bicycles, which reads in relevant part:
It shall be unlawful for any person to abandon any bicycle on any public way within the city. A bicycle shall be deemed abandoned if it: (1) is in such a state of disrepair as to be incapable of being operated in its present condition, or (2) has not been moved or used in more than seven days and bears physical indications of having been deserted.