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 $222.40   8.312%   $240.8859   $240.89   $240.90   $240.90  
  Purchase #2 $110.90   5.645%   $117.1603   $117.16   $117.15   $117.20  
  Purchase #3 $149.26   6.303%   $158.6679   $158.67   $158.65   $158.70  
  Purchase #4 $73.62   5.448%   $77.6308   $77.63   $77.65   $77.60  
  Purchase #5 $140.54   13.393%   $159.3625   $159.36   $159.35   $159.40  
  Purchase #6 $111.48   12.849%   $125.8041   $125.80   $125.80   $125.80  
  Purchase #7 $35.75   10.377%   $39.4598   $39.46   $39.45   $39.50  
  7 purchase totals = $843.95       $918.9713   $918.97   $918.95   $919.10