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 $19.75   7.264%   $21.1846   $21.18   $21.20   $21.20  
  Purchase #2 $38.91   10.229%   $42.8901   $42.89   $42.90   $42.90  
  Purchase #3 $173.61   9.146%   $189.4884   $189.49   $189.50   $189.50  
  Purchase #4 $19.94   7.388%   $21.4132   $21.41   $21.40   $21.40  
  Purchase #5 $78.64   8.149%   $85.0484   $85.05   $85.05   $85.00  
  Purchase #6 $6.44   4.778%   $6.7477   $6.75   $6.75   $6.70  
  Purchase #7 $162.34   12.706%   $182.9669   $182.97   $182.95   $183.00  
  7 purchase totals = $499.63       $549.7393   $549.74   $549.75   $549.70