app/design/adminhtml/default/default/template/sales/order/view/items/renderer/default.phtml

line number 239

app/code/core/Mage/Adminhtml/Block/Sales/Items/Abstract.php
line no 418 has below code
public function displayTaxPercent(Varien_Object $item)
{

    if ($item->getTaxPercent()) {
        return sprintf('%s%%', $item->getTaxPercent() + 0);
    } else {
        return '0%';
    }
}