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 $38.92   5.104%   $40.9065   $40.91   $40.90   $40.90  
  Purchase #2 $206.63   10.309%   $227.9315   $227.93   $227.95   $227.90  
  Purchase #3 $34.36   11.493%   $38.309   $38.31   $38.30   $38.30  
  Purchase #4 $207.65   7.120%   $222.4347   $222.43   $222.45   $222.40  
  Purchase #5 $104.70   8.545%   $113.6466   $113.65   $113.65   $113.60  
  Purchase #6 $64.30   5.404%   $67.7748   $67.77   $67.75   $67.80  
  Purchase #7 $200.86   3.395%   $207.6792   $207.68   $207.70   $207.70  
  7 purchase totals = $857.42       $918.6823   $918.68   $918.70   $918.60