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 $231.43   10.614%   $255.994   $255.99   $256.00   $256.00  
  Purchase #2 $161.32   9.250%   $176.2421   $176.24   $176.25   $176.20  
  Purchase #3 $46.76   8.649%   $50.8043   $50.80   $50.80   $50.80  
  Purchase #4 $8.64   11.390%   $9.6241   $9.62   $9.60   $9.60  
  Purchase #5 $58.64   8.511%   $63.6309   $63.63   $63.65   $63.60  
  Purchase #6 $13.71   10.448%   $15.1424   $15.14   $15.15   $15.10  
  Purchase #7 $141.53   4.225%   $147.5096   $147.51   $147.50   $147.50  
  7 purchase totals = $662.03       $718.9474   $718.93   $718.95   $718.80