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 $17.37   3.643%   $18.0028   $18.00   $18.00   $18.00  
  Purchase #2 $222.45   6.149%   $236.1285   $236.13   $236.15   $236.10  
  Purchase #3 $15.88   9.102%   $17.3254   $17.33   $17.35   $17.30  
  Purchase #4 $164.50   12.304%   $184.7401   $184.74   $184.75   $184.70  
  Purchase #5 $21.96   5.433%   $23.1531   $23.15   $23.15   $23.20  
  Purchase #6 $14.30   4.456%   $14.9372   $14.94   $14.95   $14.90  
  Purchase #7 $58.80   3.157%   $60.6563   $60.66   $60.65   $60.70  
  7 purchase totals = $515.26       $554.9434   $554.95   $555.00   $554.90