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 $56.87   13.552%   $64.577   $64.58   $64.60   $64.60  
  Purchase #2 $217.52   13.762%   $247.4551   $247.46   $247.45   $247.50  
  Purchase #3 $90.53   7.361%   $97.1939   $97.19   $97.20   $97.20  
  Purchase #4 $81.94   6.259%   $87.0686   $87.07   $87.05   $87.10  
  Purchase #5 $236.68   4.630%   $247.6383   $247.64   $247.65   $247.60  
  Purchase #6 $69.76   12.199%   $78.27   $78.27   $78.25   $78.30  
  Purchase #7 $162.61   4.127%   $169.3209   $169.32   $169.30   $169.30  
  7 purchase totals = $915.91       $991.5238   $991.53   $991.50   $991.60