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 $239.76   11.754%   $267.9414   $267.94   $267.95   $267.90  
  Purchase #2 $176.87   3.722%   $183.4531   $183.45   $183.45   $183.50  
  Purchase #3 $193.37   3.554%   $200.2424   $200.24   $200.25   $200.20  
  Purchase #4 $27.98   5.700%   $29.5749   $29.57   $29.55   $29.60  
  Purchase #5 $24.72   13.212%   $27.986   $27.99   $28.00   $28.00  
  Purchase #6 $175.36   5.811%   $185.5502   $185.55   $185.55   $185.60  
  Purchase #7 $115.43   7.132%   $123.6625   $123.66   $123.65   $123.70  
  7 purchase totals = $953.49       $1,018.4105   $1,018.40   $1,018.40   $1,018.50