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 $89.92   10.517%   $99.3769   $99.38   $99.40   $99.40  
  Purchase #2 $18.42   8.235%   $19.9369   $19.94   $19.95   $19.90  
  Purchase #3 $53.29   10.235%   $58.7442   $58.74   $58.75   $58.70  
  Purchase #4 $16.59   12.770%   $18.7085   $18.71   $18.70   $18.70  
  Purchase #5 $81.39   10.704%   $90.102   $90.10   $90.10   $90.10  
  Purchase #6 $174.76   13.183%   $197.7986   $197.80   $197.80   $197.80  
  Purchase #7 $17.60   9.340%   $19.2438   $19.24   $19.25   $19.20  
  7 purchase totals = $451.97       $503.9109   $503.91   $503.95   $503.80