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 $16.30   10.385%   $17.9928   $17.99   $18.00   $18.00  
  Purchase #2 $58.42   3.410%   $60.4121   $60.41   $60.40   $60.40  
  Purchase #3 $36.83   5.853%   $38.9857   $38.99   $39.00   $39.00  
  Purchase #4 $87.63   3.104%   $90.35   $90.35   $90.35   $90.40  
  Purchase #5 $198.42   4.538%   $207.4243   $207.42   $207.40   $207.40  
  Purchase #6 $227.81   13.234%   $257.9584   $257.96   $257.95   $258.00  
  Purchase #7 $37.69   9.670%   $41.3346   $41.33   $41.35   $41.30  
  7 purchase totals = $663.10       $714.4579   $714.45   $714.45   $714.50