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 $5.73   9.780%   $6.2904   $6.29   $6.30   $6.30  
  Purchase #2 $52.29   8.181%   $56.5678   $56.57   $56.55   $56.60  
  Purchase #3 $89.44   13.177%   $101.2255   $101.23   $101.25   $101.20  
  Purchase #4 $13.78   8.870%   $15.0023   $15.00   $15.00   $15.00  
  Purchase #5 $240.30   3.534%   $248.7922   $248.79   $248.80   $248.80  
  Purchase #6 $25.99   11.292%   $28.9248   $28.92   $28.90   $28.90  
  Purchase #7 $21.80   6.537%   $23.2251   $23.23   $23.25   $23.20  
  7 purchase totals = $449.33       $480.0281   $480.03   $480.05   $480.00