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 $50.92   11.171%   $56.6083   $56.61   $56.60   $56.60  
  Purchase #2 $224.56   12.515%   $252.6637   $252.66   $252.65   $252.70  
  Purchase #3 $36.88   10.807%   $40.8656   $40.87   $40.85   $40.90  
  Purchase #4 $229.34   8.445%   $248.7078   $248.71   $248.70   $248.70  
  Purchase #5 $113.95   11.811%   $127.4086   $127.41   $127.40   $127.40  
  Purchase #6 $249.32   12.518%   $280.5299   $280.53   $280.55   $280.50  
  Purchase #7 $224.25   10.206%   $247.137   $247.14   $247.15   $247.10  
  7 purchase totals = $1,129.22       $1,253.9209   $1,253.93   $1,253.90   $1,253.90