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 $59.44   12.105%   $66.6352   $66.64   $66.65   $66.60  
  Purchase #2 $16.33   13.589%   $18.5491   $18.55   $18.55   $18.50  
  Purchase #3 $117.92   4.298%   $122.9882   $122.99   $123.00   $123.00  
  Purchase #4 $41.97   13.244%   $47.5285   $47.53   $47.55   $47.50  
  Purchase #5 $211.47   10.237%   $233.1182   $233.12   $233.10   $233.10  
  Purchase #6 $164.67   13.502%   $186.9037   $186.90   $186.90   $186.90  
  Purchase #7 $46.29   5.721%   $48.9383   $48.94   $48.95   $48.90  
  7 purchase totals = $658.09       $724.6612   $724.67   $724.70   $724.50