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 $250.91   6.644%   $267.5805   $267.58   $267.60   $267.60  
  Purchase #2 $58.85   10.866%   $65.2446   $65.24   $65.25   $65.20  
  Purchase #3 $94.59   8.641%   $102.7635   $102.76   $102.75   $102.80  
  Purchase #4 $129.75   3.627%   $134.456   $134.46   $134.45   $134.50  
  Purchase #5 $246.25   6.314%   $261.7982   $261.80   $261.80   $261.80  
  Purchase #6 $33.63   5.796%   $35.5792   $35.58   $35.60   $35.60  
  Purchase #7 $124.63   9.822%   $136.8712   $136.87   $136.85   $136.90  
  7 purchase totals = $938.61       $1,004.2932   $1,004.29   $1,004.30   $1,004.40