As Opus 17 said, for the most benefit you want the entire gross amount of the distribution from the 401(k) to end up in the Roth IRA, and that's most easily accomplished by having no taxes withheld and using other money to pay the taxes via estimated tax payments. Tax withholding can only be avoided by doing a direct rollover. If you do have taxes withheld, you have the option to substitute other funds within 60 days to indirectly complete the rollover of the entire amount.
California permits state tax withholding to be avoided only if you have no federal taxes withheld.
