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 $244.92   6.404%   $260.6047   $260.60   $260.60   $260.60  
  Purchase #2 $134.91   8.660%   $146.5932   $146.59   $146.60   $146.60  
  Purchase #3 $36.49   12.750%   $41.1425   $41.14   $41.15   $41.10  
  Purchase #4 $232.99   10.437%   $257.3072   $257.31   $257.30   $257.30  
  Purchase #5 $184.66   7.451%   $198.419   $198.42   $198.40   $198.40  
  Purchase #6 $12.26   10.116%   $13.5002   $13.50   $13.50   $13.50  
  Purchase #7 $71.31   12.175%   $79.992   $79.99   $80.00   $80.00  
  7 purchase totals = $917.54       $997.5588   $997.55   $997.55   $997.50