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 $231.83   12.171%   $260.046   $260.05   $260.05   $260.00  
  Purchase #2 $24.86   12.129%   $27.8753   $27.88   $27.90   $27.90  
  Purchase #3 $121.57   11.830%   $135.9517   $135.95   $135.95   $136.00  
  Purchase #4 $231.34   13.825%   $263.3228   $263.32   $263.30   $263.30  
  Purchase #5 $235.68   5.719%   $249.1585   $249.16   $249.15   $249.20  
  Purchase #6 $21.78   3.159%   $22.468   $22.47   $22.45   $22.50  
  Purchase #7 $15.54   8.468%   $16.8559   $16.86   $16.85   $16.90  
  7 purchase totals = $882.60       $975.6782   $975.69   $975.65   $975.80