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 $60.31   11.211%   $67.0714   $67.07   $67.05   $67.10  
  Purchase #2 $80.52   4.285%   $83.9703   $83.97   $83.95   $84.00  
  Purchase #3 $20.46   3.509%   $21.1779   $21.18   $21.20   $21.20  
  Purchase #4 $176.77   5.592%   $186.655   $186.66   $186.65   $186.70  
  Purchase #5 $56.31   10.575%   $62.2648   $62.26   $62.25   $62.30  
  Purchase #6 $234.27   11.259%   $260.6465   $260.65   $260.65   $260.60  
  Purchase #7 $229.92   11.496%   $256.3516   $256.35   $256.35   $256.40  
  7 purchase totals = $858.56       $938.1375   $938.14   $938.10   $938.30