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 $19.56   7.235%   $20.9752   $20.98   $21.00   $21.00  
  Purchase #2 $100.81   3.708%   $104.548   $104.55   $104.55   $104.50  
  Purchase #3 $51.93   9.753%   $56.9947   $56.99   $57.00   $57.00  
  Purchase #4 $10.57   4.166%   $11.0103   $11.01   $11.00   $11.00  
  Purchase #5 $51.44   9.651%   $56.4045   $56.40   $56.40   $56.40  
  Purchase #6 $39.41   6.699%   $42.0501   $42.05   $42.05   $42.10  
  Purchase #7 $232.86   12.353%   $261.6252   $261.63   $261.65   $261.60  
  7 purchase totals = $506.58       $553.6080   $553.61   $553.65   $553.60