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 $18.89   12.720%   $21.2928   $21.29   $21.30   $21.30  
  Purchase #2 $10.78   4.495%   $11.2646   $11.26   $11.25   $11.30  
  Purchase #3 $123.82   10.578%   $136.9177   $136.92   $136.90   $136.90  
  Purchase #4 $127.93   8.871%   $139.2787   $139.28   $139.30   $139.30  
  Purchase #5 $18.84   3.683%   $19.5339   $19.53   $19.55   $19.50  
  Purchase #6 $127.44   3.838%   $132.3311   $132.33   $132.35   $132.30  
  Purchase #7 $122.53   9.796%   $134.533   $134.53   $134.55   $134.50  
  7 purchase totals = $550.23       $595.1518   $595.14   $595.20   $595.10