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 $17.82   4.212%   $18.5706   $18.57   $18.55   $18.60  
  Purchase #2 $94.95   9.223%   $103.7072   $103.71   $103.70   $103.70  
  Purchase #3 $71.63   8.272%   $77.5552   $77.56   $77.55   $77.60  
  Purchase #4 $105.70   7.314%   $113.4309   $113.43   $113.45   $113.40  
  Purchase #5 $28.45   7.434%   $30.565   $30.57   $30.55   $30.60  
  Purchase #6 $21.55   3.357%   $22.2734   $22.27   $22.25   $22.30  
  Purchase #7 $230.97   8.793%   $251.2792   $251.28   $251.30   $251.30  
  7 purchase totals = $571.07       $617.3815   $617.39   $617.35   $617.50