As Bsch4477 said, default withholding is 10% of the distribution. However, because a trust cannot pass through ordinary withholding to trust beneficiaries, if the income will be passed through to trust beneficiaries on Schedules K-1 (Form 1041), tax withholding should be declined. Only if the trust will have a tax liability (which would not be the case if all trust income is passed through to trust beneficiaries) would it make any sense to have taxes withheld from the distribution.
With income being passed through to trust beneficiaries, the trust beneficiaries will have to have sufficient taxes withheld from other sources or make estimated tax payments themselves sufficient to avoid any tax underpayment penalty. You'll want to inform the beneficiaries of the amount of income that will be passed through to them on the Schedules K-1 so that they can do a proper estimate of their tax liabilities.
