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 $235.28   11.872%   $263.2124   $263.21   $263.20   $263.20  
  Purchase #2 $124.71   8.135%   $134.8552   $134.86   $134.85   $134.90  
  Purchase #3 $12.31   5.549%   $12.9931   $12.99   $13.00   $13.00  
  Purchase #4 $154.66   8.599%   $167.9592   $167.96   $167.95   $168.00  
  Purchase #5 $111.27   9.637%   $121.9931   $121.99   $122.00   $122.00  
  Purchase #6 $224.48   12.521%   $252.5871   $252.59   $252.60   $252.60  
  Purchase #7 $6.73   9.782%   $7.3883   $7.39   $7.40   $7.40  
  7 purchase totals = $869.44       $960.9884   $960.99   $961.00   $961.10