My experience that I’ve had with this is different with each airport. For O’Hare airport I don’t get bothered too much, but I have had bad experiences with O’Hare and Rockford Airports.
A great spotting airport around my area (Chicago) is Chicago Regional airport. At the south of the airport there is an observation area for spotters. Also at Milwaukee Mitchell airport there is a spotter parking lot on the north end of the field.
Just my advice is if you go to an airport to watch and photograph airplanes make sure you’re on PUBLIC property. DO NOT TRESPASS!!!