I created a purchase order (PO) for repairs/maintenance services from a supplier using an amount-based form. But there may also be parts/supplies that are taxable. Should I mark the entire PO as taxable?

You should not mark the whole PO as taxable. You can create your PO using two lines. One line will be for the repair/maintenance services that are non-taxable. On this line, ensure the Taxable checkbox is unchecked. The second PO line will be for the parts/supplies and the Taxable checkbox needs to be checked. This PO setup will ensure that the appropriate amount of tax is paid, the lien (encumbrance) amount includes tax, and helps prevent invoicing delays.