The basic guidance from the IRS and subsequent case law is that gifts provided as recognition of or compensation for services provided are taxable income. Most gifts from the congregation to staff will therefore be treated as taxable income and included on the individual's W-2. If the congregation participates in the UUA retirement plan, cash gifts should also be included in calculating that remittance.
If gifts are the result of an offering or collection, such contributions would not be considered tax-deductible for the donor.