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 $236.87   12.248%   $265.8818   $265.88   $265.90   $265.90  
  Purchase #2 $158.94   11.127%   $176.6253   $176.63   $176.65   $176.60  
  Purchase #3 $19.50   10.155%   $21.4802   $21.48   $21.50   $21.50  
  Purchase #4 $103.63   10.338%   $114.3433   $114.34   $114.35   $114.30  
  Purchase #5 $103.65   13.274%   $117.4085   $117.41   $117.40   $117.40  
  Purchase #6 $116.42   4.131%   $121.2293   $121.23   $121.25   $121.20  
  Purchase #7 $157.62   8.642%   $171.2415   $171.24   $171.25   $171.20  
  7 purchase totals = $896.63       $988.2099   $988.21   $988.30   $988.10