If you reside in the United States, you cannot dodge the issue of state residency and tax domicile. Each state has its definition of residency and taxation of income. If you are not careful, more than one state might claim you as a resident and tax your income accordingly.
California can be tough on residency determination. Whether you are determined to be a resident or non-resident of California, your income earned in California will be subject to taxation by the state. If a resident, then all your worldwide income will be subject to California income tax. See the following passages and other information from FTB Publication 1031 at:
https://www.ftb.ca.gov/forms/2011/11_1031.pdf
"Generally, your state of residence is where you have your closest connections. If you leave your state of residence, it is important to determine if your presence in a different location is for a temporary or transitory purpose. You should consider the purpose and length of your stay when determining your residency."
"When you are present in California for temporary or transitory purposes, you are a nonresident of California. For instance, if you come to California for a vacation, or to complete a transaction, or are simply passing through, your purpose is temporary or transitory. As a nonresident, you are taxed only on your income from California sources."
"When you are in California for other than a temporary or transitory purpose, you are a California resident. . For instance, if your employer assigns you to an office in California for a long or indefinite period, if you retire and come to California with no specific plans to leave, or if you are ill and are in California for an indefinite recuperation period, your stay is other than temporary or transitory. As a resident, you are taxed on income from all sources. You will be presumed to be a California resident for any taxable year in which you spend more than nine months in this state."
"Although you may have connections with another state, if your stay in California is for other than a temporary or transitory purpose, you are a California resident. As a resident, your income from all sources is taxable by California."
