It's not required by Kip and friends. However, I usually put the liquids that are in checked baggage in zip lock bags (when I check luggage). Checked luggage experiences changes in air pressure which can cause liquids to leak from their containers. This can cause an unpleasant surprise when one arrives at one's destination. I put any liquids I check in zip lock bags to prevent this. Also, if it's in a sealed zip lock bag, the liquid that leaked can usually be easily recovered and returned to it's container upon arrival.