If you deliberately try to collect rainwater through clandestine means, and you get caught, it is then elevated to fraud (due to intention). That would then equate to a prison sentence if you get caught.

Not worth it.

Thinking about storing it underground? Better hope your local water enforcement agency doesn't hire a helicopter with a flir system, which can identify underground water sources by their infrared footprint while flying overhead. Maybe you can dig a hole deep enough that the thermal variance will get masked. Of course there's the building code inspectors that might notice the constructive efforts on your property and want to find out what's going on. Have you filed a permit app for that underground tank on your property? Is there a new outbuilding that just popped up that isn't in last year's overhead picture? Or maybe they'll just look for typical usage indicators. Are you planning on watering your lawn with that water when your neighbors yard is brown? Will the sewer camera see a greater outfall from your service connection than the rest of the neighborhhod?

Having the water is not so important as being able to use it when needed. Unfortunately, such uses are easy to detect, if you know where to look, and they are getting very good at recognizing these things.