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 $37.53   4.269%   $39.1322   $39.13   $39.15   $39.10  
  Purchase #2 $75.30   4.241%   $78.4935   $78.49   $78.50   $78.50  
  Purchase #3 $222.70   6.281%   $236.6878   $236.69   $236.70   $236.70  
  Purchase #4 $70.56   8.630%   $76.6493   $76.65   $76.65   $76.60  
  Purchase #5 $19.40   8.415%   $21.0325   $21.03   $21.05   $21.00  
  Purchase #6 $92.77   10.459%   $102.4728   $102.47   $102.45   $102.50  
  Purchase #7 $229.32   12.210%   $257.32   $257.32   $257.30   $257.30  
  7 purchase totals = $747.58       $811.7881   $811.78   $811.80   $811.70