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 $51.89   13.662%   $58.9792   $58.98   $59.00   $59.00  
  Purchase #2 $138.41   10.192%   $152.5167   $152.52   $152.50   $152.50  
  Purchase #3 $52.54   12.128%   $58.9121   $58.91   $58.90   $58.90  
  Purchase #4 $138.40   8.128%   $149.6492   $149.65   $149.65   $149.60  
  Purchase #5 $137.38   6.291%   $146.0226   $146.02   $146.00   $146.00  
  Purchase #6 $97.46   13.153%   $110.2789   $110.28   $110.30   $110.30  
  Purchase #7 $236.54   10.531%   $261.45   $261.45   $261.45   $261.50  
  7 purchase totals = $852.62       $937.8087   $937.81   $937.80   $937.80