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 $229.95   13.653%   $261.3451   $261.35   $261.35   $261.30  
  Purchase #2 $239.42   12.241%   $268.7274   $268.73   $268.75   $268.70  
  Purchase #3 $59.49   3.181%   $61.3824   $61.38   $61.40   $61.40  
  Purchase #4 $46.44   6.786%   $49.5914   $49.59   $49.60   $49.60  
  Purchase #5 $79.99   5.669%   $84.5246   $84.52   $84.50   $84.50  
  Purchase #6 $71.91   7.296%   $77.1566   $77.16   $77.15   $77.20  
  Purchase #7 $27.83   5.214%   $29.2811   $29.28   $29.30   $29.30  
  7 purchase totals = $755.03       $832.0086   $832.01   $832.05   $832.00