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 $20.63   3.677%   $21.3886   $21.39   $21.40   $21.40  
  Purchase #2 $115.99   5.859%   $122.7859   $122.79   $122.80   $122.80  
  Purchase #3 $138.42   13.818%   $157.5469   $157.55   $157.55   $157.50  
  Purchase #4 $85.62   12.577%   $96.3884   $96.39   $96.40   $96.40  
  Purchase #5 $194.51   13.476%   $220.7222   $220.72   $220.70   $220.70  
  Purchase #6 $6.58   12.685%   $7.4147   $7.41   $7.40   $7.40  
  Purchase #7 $249.51   6.379%   $265.4262   $265.43   $265.45   $265.40  
  7 purchase totals = $811.26       $891.6729   $891.68   $891.70   $891.60