Unwanted pre-rounding in admin's zen_add_tax function

[lat9]

The admin version of zen_add_tax (as far back as zc154) performs unwanted pre-rounding of the calculated result, unlike the storefront version:

/**
 * Add tax to a products price
 */
  function zen_add_tax($price, $tax) {
    global $currencies;

    if (DISPLAY_PRICE_WITH_TAX_ADMIN == 'true') {
      return zen_round($price, $currencies->currencies[DEFAULT_CURRENCY]['decimal_places']) + zen_calculate_tax($price, $tax);
    } else {
      return zen_round($price, $currencies->currencies[DEFAULT_CURRENCY]['decimal_places']);
    }
  }

I believe that this is one of the causes of many rounding issues seen by Edit Orders when a store is configured to display prices with tax in the admin.