I am not a scofflaw, neither am I worried about such inconsequential issues such as this. I'm very much in line with PawPaw_n_Gram and donn0128 in just make sure you have it insured, BOS and prior registration.
I have never heard of anyone being stopped just to check registration. It is possible if you do not have the plates from the sellers state that you could be stopped but not too likely that you would even be stopped if no plates.
In CA, you leave the plates on a vehicle unless they are vanity plates. No big deal anyplace.
