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 $240.95   11.422%   $268.4713   $268.47   $268.45   $268.50  
  Purchase #2 $222.97   4.341%   $232.6491   $232.65   $232.65   $232.60  
  Purchase #3 $81.57   3.233%   $84.2072   $84.21   $84.20   $84.20  
  Purchase #4 $85.90   11.169%   $95.4942   $95.49   $95.50   $95.50  
  Purchase #5 $76.28   5.508%   $80.4815   $80.48   $80.50   $80.50  
  Purchase #6 $17.84   8.240%   $19.31   $19.31   $19.30   $19.30  
  Purchase #7 $220.94   8.337%   $239.3598   $239.36   $239.35   $239.40  
  7 purchase totals = $946.45       $1,019.9731   $1,019.97   $1,019.95   $1,020.00