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 $20.47   6.606%   $21.8222   $21.82   $21.80   $21.80  
  Purchase #2 $188.64   4.426%   $196.9892   $196.99   $197.00   $197.00  
  Purchase #3 $181.86   5.839%   $192.4788   $192.48   $192.50   $192.50  
  Purchase #4 $125.50   12.704%   $141.4435   $141.44   $141.45   $141.40  
  Purchase #5 $108.69   6.259%   $115.4929   $115.49   $115.50   $115.50  
  Purchase #6 $6.29   9.863%   $6.9104   $6.91   $6.90   $6.90  
  Purchase #7 $12.66   6.493%   $13.482   $13.48   $13.50   $13.50  
  7 purchase totals = $644.11       $688.6190   $688.61   $688.65   $688.60