Upon further thought, whatever you choose to do, make sure that it doesn't subject you to sales tax in that state (except CA, because you can't get out of that).
Oh yes, and when you register your trailer in CA, you will have to bring it to the DMV and show them the registration information on the trailer itself, at least that's what we had to do when we registered ours.
I just checked the CA DMV site and it says you only have 20 days to register (or pay the fees):
http://www.dmv.ca.gov/pubs/brochures/howto/htvr9.htm