Most local police don't care about minor vehicle code violations on classic cars unless they're blatant and unsafe. Even then, you're more likely to be stopped and receive a citation from a motor(cycle) officer or a CHP officer, since they primarily enforce traffic laws. Regular patrol officers in local jurisdictions use violations they observe as a reason to stop you and check for other things (driver license status, warrants, etc), but will rarely issue a citation for equipment violations unless the violation is really an issue. Basically, the patrol officers are looking for bigger fish to fry than to issue a ticket for ride height.
Anyway, as for lowered cars, I really wouldn't worry too much about it. Unless you have turndown exhaust making it really noisy, you probably won't be stopped for anything related to the car being low. Drivers of lifted trucks need to worry more because they have more common violations to worry about (lack of mud flaps, tire tread, headlight height, beam alignment, etc.).
I should mention that these are simply my experiences from working in law enforcement around the SF bay area of California. Other areas may differ a bit, but I think this is true for the most part. If you cruise past me, you're more likely to get a thumbs up than a ticket for your lowered ride.