When reimbursing employees for business expenses, is there a threshold amount under which a receipt is not technically required by the IRS?

The reimbursement can be under accountable plan or nonaccountable plan. Under the accountable plan, you must get receipt from the employee and make sure the expense is actual business expense (substantiated expense).

Under non-accountable plan, you do not need actual receipts and you report the payment on W-2 or 1099-misc.