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 $59.49   12.284%   $66.7978   $66.80   $66.80   $66.80  
  Purchase #2 $121.46   10.813%   $134.5935   $134.59   $134.60   $134.60  
  Purchase #3 $88.82   9.615%   $97.36   $97.36   $97.35   $97.40  
  Purchase #4 $238.27   9.763%   $261.5323   $261.53   $261.55   $261.50  
  Purchase #5 $75.97   8.631%   $82.527   $82.53   $82.55   $82.50  
  Purchase #6 $218.56   10.481%   $241.4673   $241.47   $241.45   $241.50  
  Purchase #7 $123.78   10.181%   $136.382   $136.38   $136.40   $136.40  
  7 purchase totals = $926.35       $1,020.6599   $1,020.66   $1,020.70   $1,020.70