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 $118.55   13.216%   $134.2176   $134.22   $134.20   $134.20  
  Purchase #2 $83.50   12.150%   $93.6453   $93.65   $93.65   $93.60  
  Purchase #3 $21.80   7.193%   $23.3681   $23.37   $23.35   $23.40  
  Purchase #4 $115.93   10.328%   $127.9033   $127.90   $127.90   $127.90  
  Purchase #5 $104.91   8.557%   $113.8871   $113.89   $113.90   $113.90  
  Purchase #6 $75.42   8.333%   $81.7047   $81.70   $81.70   $81.70  
  Purchase #7 $18.77   3.763%   $19.4763   $19.48   $19.50   $19.50  
  7 purchase totals = $538.88       $594.2024   $594.21   $594.20   $594.20