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 $240.99   8.122%   $260.5632   $260.56   $260.55   $260.60  
  Purchase #2 $113.87   4.861%   $119.4052   $119.41   $119.40   $119.40  
  Purchase #3 $131.48   6.682%   $140.2655   $140.27   $140.25   $140.30  
  Purchase #4 $163.39   4.251%   $170.3357   $170.34   $170.35   $170.30  
  Purchase #5 $109.64   5.443%   $115.6077   $115.61   $115.60   $115.60  
  Purchase #6 $118.37   3.425%   $122.4242   $122.42   $122.40   $122.40  
  Purchase #7 $61.80   3.182%   $63.7665   $63.77   $63.75   $63.80  
  7 purchase totals = $939.54       $992.3680   $992.38   $992.30   $992.40