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 $211.31   13.568%   $239.9805   $239.98   $240.00   $240.00  
  Purchase #2 $118.35   12.119%   $132.6928   $132.69   $132.70   $132.70  
  Purchase #3 $76.90   5.251%   $80.938   $80.94   $80.95   $80.90  
  Purchase #4 $107.82   9.856%   $118.4467   $118.45   $118.45   $118.40  
  Purchase #5 $35.78   7.323%   $38.4002   $38.40   $38.40   $38.40  
  Purchase #6 $124.47   13.255%   $140.9685   $140.97   $140.95   $141.00  
  Purchase #7 $133.54   11.482%   $148.8731   $148.87   $148.85   $148.90  
  7 purchase totals = $808.17       $900.2998   $900.30   $900.30   $900.30