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 $249.68   13.741%   $283.9885   $283.99   $284.00   $284.00  
  Purchase #2 $224.81   12.819%   $253.6284   $253.63   $253.65   $253.60  
  Purchase #3 $164.32   4.807%   $172.2189   $172.22   $172.20   $172.20  
  Purchase #4 $47.87   8.327%   $51.8561   $51.86   $51.85   $51.90  
  Purchase #5 $247.59   13.174%   $280.2075   $280.21   $280.20   $280.20  
  Purchase #6 $247.30   3.677%   $256.3932   $256.39   $256.40   $256.40  
  Purchase #7 $114.37   4.379%   $119.3783   $119.38   $119.40   $119.40  
  7 purchase totals = $1,295.94       $1,417.6709   $1,417.68   $1,417.70   $1,417.70