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 $13.34   13.328%   $15.118   $15.12   $15.10   $15.10  
  Purchase #2 $31.61   7.535%   $33.9918   $33.99   $34.00   $34.00  
  Purchase #3 $229.95   13.301%   $260.5356   $260.54   $260.55   $260.50  
  Purchase #4 $229.41   8.613%   $249.1691   $249.17   $249.15   $249.20  
  Purchase #5 $143.44   3.715%   $148.7688   $148.77   $148.75   $148.80  
  Purchase #6 $239.25   7.765%   $257.8278   $257.83   $257.85   $257.80  
  Purchase #7 $140.34   4.375%   $146.4799   $146.48   $146.50   $146.50  
  7 purchase totals = $1,027.34       $1,111.8910   $1,111.90   $1,111.90   $1,111.90