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 $234.57   10.532%   $259.2749   $259.27   $259.25   $259.30  
  Purchase #2 $170.77   6.680%   $182.1774   $182.18   $182.20   $182.20  
  Purchase #3 $98.42   4.861%   $103.2042   $103.20   $103.20   $103.20  
  Purchase #4 $240.98   5.622%   $254.5279   $254.53   $254.55   $254.50  
  Purchase #5 $196.59   3.732%   $203.9267   $203.93   $203.95   $203.90  
  Purchase #6 $68.46   9.291%   $74.8206   $74.82   $74.80   $74.80  
  Purchase #7 $233.73   6.738%   $249.4787   $249.48   $249.50   $249.50  
  7 purchase totals = $1,243.52       $1,327.4104   $1,327.41   $1,327.45   $1,327.40