If it is a taxable benefit, is income tax always withheld on the contribution?
No single withholding answer follows from the taxable-benefit label. Under CRA's cash-benefit treatment, the employer may use the income-tax withholding exception when it has reasonable grounds to believe the employee can deduct the RRSP contribution for that year. CRA's non-cash-benefit treatment lists income-tax withholding without stating that exception. The employer needs to confirm the benefit form, conditions and evidence. Other payroll questions remain separate.