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 $59.95   10.110%   $66.0109   $66.01   $66.00   $66.00  
  Purchase #2 $139.26   5.427%   $146.8176   $146.82   $146.80   $146.80  
  Purchase #3 $231.62   11.141%   $257.4248   $257.42   $257.40   $257.40  
  Purchase #4 $116.81   11.107%   $129.7841   $129.78   $129.80   $129.80  
  Purchase #5 $233.63   5.403%   $246.253   $246.25   $246.25   $246.30  
  Purchase #6 $66.82   5.672%   $70.61   $70.61   $70.60   $70.60  
  Purchase #7 $69.71   10.479%   $77.0149   $77.01   $77.00   $77.00  
  7 purchase totals = $917.80       $993.9153   $993.90   $993.85   $993.90