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 $67.32   13.402%   $76.3422   $76.34   $76.35   $76.30  
  Purchase #2 $61.37   5.667%   $64.8478   $64.85   $64.85   $64.80  
  Purchase #3 $99.89   7.509%   $107.3907   $107.39   $107.40   $107.40  
  Purchase #4 $245.94   10.812%   $272.531   $272.53   $272.55   $272.50  
  Purchase #5 $56.32   3.598%   $58.3464   $58.35   $58.35   $58.30  
  Purchase #6 $125.64   6.517%   $133.828   $133.83   $133.85   $133.80  
  Purchase #7 $93.50   12.779%   $105.4484   $105.45   $105.45   $105.40  
  7 purchase totals = $749.98       $818.7345   $818.74   $818.80   $818.50