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 $28.80   5.120%   $30.2746   $30.27   $30.25   $30.30  
  Purchase #2 $70.71   13.701%   $80.398   $80.40   $80.40   $80.40  
  Purchase #3 $199.73   8.630%   $216.9667   $216.97   $216.95   $217.00  
  Purchase #4 $123.57   5.325%   $130.1501   $130.15   $130.15   $130.20  
  Purchase #5 $76.64   4.249%   $79.8964   $79.90   $79.90   $79.90  
  Purchase #6 $80.26   7.176%   $86.0195   $86.02   $86.00   $86.00  
  Purchase #7 $15.78   5.105%   $16.5856   $16.59   $16.60   $16.60  
  7 purchase totals = $595.49       $640.2909   $640.30   $640.25   $640.40