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 $16.44   9.508%   $18.0031   $18.00   $18.00   $18.00  
  Purchase #2 $72.32   9.513%   $79.1998   $79.20   $79.20   $79.20  
  Purchase #3 $142.77   10.870%   $158.2891   $158.29   $158.30   $158.30  
  Purchase #4 $207.87   4.736%   $217.7147   $217.71   $217.70   $217.70  
  Purchase #5 $22.28   4.166%   $23.2082   $23.21   $23.20   $23.20  
  Purchase #6 $95.54   12.238%   $107.2322   $107.23   $107.25   $107.20  
  Purchase #7 $80.38   6.210%   $85.3716   $85.37   $85.35   $85.40  
  7 purchase totals = $637.60       $689.0187   $689.01   $689.00   $689.00