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 $77.76   5.692%   $82.1861   $82.19   $82.20   $82.20  
  Purchase #2 $90.38   3.441%   $93.49   $93.49   $93.50   $93.50  
  Purchase #3 $231.81   9.138%   $252.9928   $252.99   $253.00   $253.00  
  Purchase #4 $8.87   5.576%   $9.3646   $9.36   $9.35   $9.40  
  Purchase #5 $228.80   9.857%   $251.3528   $251.35   $251.35   $251.40  
  Purchase #6 $31.57   8.786%   $34.3437   $34.34   $34.35   $34.30  
  Purchase #7 $57.64   4.115%   $60.0119   $60.01   $60.00   $60.00  
  7 purchase totals = $726.83       $783.7419   $783.73   $783.75   $783.80