New cars are so common in CA that if the car is a late model and looks reasonably good, you'll never have a problem. Parked on the street, they can't see the dates on the registration that's stuck to the windshield. Just make sure your registration is paid and your insurance is up to date.
i had a personalized license plate on the back that i no longer cared about and i discovered that when i told the dmv that i lost the front plate (which was true, since the car had no front plate for 6 years outside of chicago) they gave me a temporary plate while the replacements were being made, which cost me all of about $10.
the temporary plate expired in 6 months, so for 6 months i was able to avoid having a front plate. if i had kept the car for longer, i would have just told them the plates never came in the mail or something, paid the small replacement fee again, and gotten another 6 month temporary plate.
otoh, a friend got a ticket for missing front plates.
If you get stopped for something else, you'll get ticketed. It's a fix-it ticket though and costs around $25 and two minutes to get signed off.