Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $68.49   13.397%   $77.6656   $77.67   $77.65   $77.70  
  Purchase #2 $224.70   11.418%   $250.3562   $250.36   $250.35   $250.40  
  Purchase #3 $71.69   3.294%   $74.0515   $74.05   $74.05   $74.10  
  Purchase #4 $74.68   4.845%   $78.2982   $78.30   $78.30   $78.30  
  Purchase #5 $203.37   12.403%   $228.594   $228.59   $228.60   $228.60  
  Purchase #6 $70.97   4.507%   $74.1686   $74.17   $74.15   $74.20  
  Purchase #7 $27.40   4.436%   $28.6155   $28.62   $28.60   $28.60  
  7 purchase totals = $741.30       $811.7496   $811.76   $811.70   $811.90