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 $40.32   4.151%   $41.9937   $41.99   $42.00   $42.00  
  Purchase #2 $99.40   12.445%   $111.7703   $111.77   $111.75   $111.80  
  Purchase #3 $138.51   3.863%   $143.8606   $143.86   $143.85   $143.90  
  Purchase #4 $133.29   10.854%   $147.7573   $147.76   $147.75   $147.80  
  Purchase #5 $129.63   3.659%   $134.3732   $134.37   $134.35   $134.40  
  Purchase #6 $250.72   7.570%   $269.6995   $269.70   $269.70   $269.70  
  Purchase #7 $67.84   3.455%   $70.1839   $70.18   $70.20   $70.20  
  7 purchase totals = $859.71       $919.6385   $919.63   $919.60   $919.80