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 $28.64   4.829%   $30.023   $30.02   $30.00   $30.00  
  Purchase #2 $200.53   10.523%   $221.6318   $221.63   $221.65   $221.60  
  Purchase #3 $148.60   4.602%   $155.4386   $155.44   $155.45   $155.40  
  Purchase #4 $151.76   8.329%   $164.4001   $164.40   $164.40   $164.40  
  Purchase #5 $133.99   8.837%   $145.8307   $145.83   $145.85   $145.80  
  Purchase #6 $51.99   11.866%   $58.1591   $58.16   $58.15   $58.20  
  Purchase #7 $108.67   5.356%   $114.4904   $114.49   $114.50   $114.50  
  7 purchase totals = $824.18       $889.9737   $889.97   $890.00   $889.90