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 $18.81   11.571%   $20.9865   $20.99   $21.00   $21.00  
  Purchase #2 $46.49   7.456%   $49.9563   $49.96   $49.95   $50.00  
  Purchase #3 $141.91   12.203%   $159.2273   $159.23   $159.25   $159.20  
  Purchase #4 $69.32   13.239%   $78.4973   $78.50   $78.50   $78.50  
  Purchase #5 $207.51   8.837%   $225.8477   $225.85   $225.85   $225.80  
  Purchase #6 $113.28   12.462%   $127.397   $127.40   $127.40   $127.40  
  Purchase #7 $61.90   9.576%   $67.8275   $67.83   $67.85   $67.80  
  7 purchase totals = $659.22       $729.7396   $729.76   $729.80   $729.70