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 $236.25   12.610%   $266.0411   $266.04   $266.05   $266.00  
  Purchase #2 $31.48   6.380%   $33.4884   $33.49   $33.50   $33.50  
  Purchase #3 $230.50   8.324%   $249.6868   $249.69   $249.70   $249.70  
  Purchase #4 $59.92   5.834%   $63.4157   $63.42   $63.40   $63.40  
  Purchase #5 $27.77   6.621%   $29.6087   $29.61   $29.60   $29.60  
  Purchase #6 $71.41   6.785%   $76.2552   $76.26   $76.25   $76.30  
  Purchase #7 $241.71   5.420%   $254.8107   $254.81   $254.80   $254.80  
  7 purchase totals = $899.04       $973.3066   $973.32   $973.30   $973.30