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 $18.99   11.704%   $21.2126   $21.21   $21.20   $21.20  
  Purchase #2 $186.48   10.223%   $205.5439   $205.54   $205.55   $205.50  
  Purchase #3 $148.26   10.154%   $163.3143   $163.31   $163.30   $163.30  
  Purchase #4 $204.42   12.468%   $229.9071   $229.91   $229.90   $229.90  
  Purchase #5 $225.62   5.840%   $238.7962   $238.80   $238.80   $238.80  
  Purchase #6 $34.83   8.806%   $37.8971   $37.90   $37.90   $37.90  
  Purchase #7 $49.72   10.359%   $54.8705   $54.87   $54.85   $54.90  
  7 purchase totals = $868.32       $951.5417   $951.54   $951.50   $951.50