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 $16.88   6.859%   $18.0378   $18.04   $18.05   $18.00  
  Purchase #2 $11.36   4.451%   $11.8656   $11.87   $11.85   $11.90  
  Purchase #3 $56.99   12.662%   $64.2061   $64.21   $64.20   $64.20  
  Purchase #4 $92.48   11.126%   $102.7693   $102.77   $102.75   $102.80  
  Purchase #5 $22.65   4.828%   $23.7435   $23.74   $23.75   $23.70  
  Purchase #6 $173.81   9.357%   $190.0734   $190.07   $190.05   $190.10  
  Purchase #7 $136.93   7.699%   $147.4722   $147.47   $147.45   $147.50  
  7 purchase totals = $511.10       $558.1679   $558.17   $558.10   $558.20