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 $247.85   13.223%   $280.6232   $280.62   $280.60   $280.60  
  Purchase #2 $196.69   4.178%   $204.9077   $204.91   $204.90   $204.90  
  Purchase #3 $22.76   8.544%   $24.7046   $24.70   $24.70   $24.70  
  Purchase #4 $69.54   8.243%   $75.2722   $75.27   $75.25   $75.30  
  Purchase #5 $184.71   4.363%   $192.7689   $192.77   $192.75   $192.80  
  Purchase #6 $236.69   5.387%   $249.4405   $249.44   $249.45   $249.40  
  Purchase #7 $165.54   10.808%   $183.4316   $183.43   $183.45   $183.40  
  7 purchase totals = $1,123.78       $1,211.1487   $1,211.14   $1,211.10   $1,211.10