I dunno if it's a law or not but I know that if you lower your car so the wheel is sticking into the fender it's considered no longer safe for the road and it will get towed away. Other than that I haven't really heard or come across anyone getting a ticket for having their car too low.
There was an article about this in SCC. Just TRY and figure out your local laws...it won't matter in the end. Some of the officers they talked to said the bottom of the car could not be lower than the center of the wheel. Too bad almost every vehicle comes like this bone stock. Another one is that the vehicle is too low if the bottom of the vehicle is below the bottom of the wheel. Another is that the headlights and foglights must be x inches above the ground. I'm sure this varies by locale.
I heard some interpretation of some regulation (California reg?) that stated that the bottom of the car can't be lower than the bottom edge of the rims. Simply put, the lower the tire profile, the lower your car can be (legally).