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 $13.30   4.540%   $13.9038   $13.90   $13.90   $13.90  
  Purchase #2 $69.91   8.471%   $75.8321   $75.83   $75.85   $75.80  
  Purchase #3 $6.99   13.526%   $7.9355   $7.94   $7.95   $7.90  
  Purchase #4 $98.46   7.186%   $105.5353   $105.54   $105.55   $105.50  
  Purchase #5 $197.76   10.679%   $218.8788   $218.88   $218.90   $218.90  
  Purchase #6 $107.60   8.317%   $116.5491   $116.55   $116.55   $116.50  
  Purchase #7 $139.61   5.110%   $146.7441   $146.74   $146.75   $146.70  
  7 purchase totals = $633.63       $685.3787   $685.38   $685.45   $685.20