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 $124.79   10.680%   $138.1176   $138.12   $138.10   $138.10  
  Purchase #2 $81.80   12.581%   $92.0913   $92.09   $92.10   $92.10  
  Purchase #3 $152.65   3.331%   $157.7348   $157.73   $157.75   $157.70  
  Purchase #4 $44.95   12.341%   $50.4973   $50.50   $50.50   $50.50  
  Purchase #5 $149.92   7.356%   $160.9481   $160.95   $160.95   $160.90  
  Purchase #6 $14.53   4.421%   $15.1724   $15.17   $15.15   $15.20  
  Purchase #7 $202.28   5.872%   $214.1579   $214.16   $214.15   $214.20  
  7 purchase totals = $770.92       $828.7194   $828.72   $828.70   $828.70