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 $51.79   8.204%   $56.0389   $56.04   $56.05   $56.00  
  Purchase #2 $107.81   6.217%   $114.5125   $114.51   $114.50   $114.50  
  Purchase #3 $8.47   6.609%   $9.0298   $9.03   $9.05   $9.00  
  Purchase #4 $175.75   8.239%   $190.23   $190.23   $190.25   $190.20  
  Purchase #5 $119.34   9.826%   $131.0663   $131.07   $131.05   $131.10  
  Purchase #6 $65.52   11.207%   $72.8628   $72.86   $72.85   $72.90  
  Purchase #7 $27.46   11.116%   $30.5125   $30.51   $30.50   $30.50  
  7 purchase totals = $556.14       $604.2528   $604.25   $604.25   $604.20