I just found this on the IRS website:
For federal income tax purposes, a single-member LLC classified as a disregarded entity generally must use the owner's social security number (SSN) or employer identification number (EIN) for all information returns and reporting related to income tax. For example, if a disregarded entity LLC that is owned by an individual is required to provide a Form W-9, Request for Taxpayer Identification Number (TIN) and Certification, the W-9 should provide the owner’s SSN or EIN, not the LLC’s EIN.
This implies I should use my spouse's SSN on the W-9 not the LLC's EIN since she is the owner of the LLC.
