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 $8.90   5.708%   $9.408   $9.41   $9.40   $9.40  
  Purchase #2 $8.50   8.844%   $9.2517   $9.25   $9.25   $9.30  
  Purchase #3 $241.40   8.221%   $261.2455   $261.25   $261.25   $261.20  
  Purchase #4 $126.74   10.469%   $140.0084   $140.01   $140.00   $140.00  
  Purchase #5 $127.56   11.861%   $142.6899   $142.69   $142.70   $142.70  
  Purchase #6 $211.58   7.734%   $227.9436   $227.94   $227.95   $227.90  
  Purchase #7 $182.93   13.656%   $207.9109   $207.91   $207.90   $207.90  
  7 purchase totals = $907.61       $998.4580   $998.46   $998.45   $998.40