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 $230.42   13.463%   $261.4414   $261.44   $261.45   $261.40  
  Purchase #2 $34.56   9.157%   $37.7247   $37.72   $37.70   $37.70  
  Purchase #3 $116.97   8.335%   $126.7194   $126.72   $126.70   $126.70  
  Purchase #4 $33.30   12.858%   $37.5817   $37.58   $37.60   $37.60  
  Purchase #5 $230.61   13.865%   $262.5841   $262.58   $262.60   $262.60  
  Purchase #6 $101.99   11.434%   $113.6515   $113.65   $113.65   $113.70  
  Purchase #7 $137.56   10.269%   $151.686   $151.69   $151.70   $151.70  
  7 purchase totals = $885.41       $991.3888   $991.38   $991.40   $991.40