sw_guy is correct.

I suggest this approach, which saves some CPU cycles and database queries:

/includes/modules/order_total/ot_group_pricing.php
around line 67 you'll see this section of code.
Add the line highlighted:
Code:
  function calculate_deductions($order_total) {
    global $db, $order;
    $od_amount = array();
    if ($order_total == 0) return $od_amount;
    $orderTotal = $this->get_order_total();