I am not a big fan of cassis or cocktails liqueur making process but Lowlander made a good point. Bacteria do exist and we should always sterilise everything we use. First of all, because we can make ourselves in danger but also we must always be honest towards our guests or relatives when preparing something, a minimum of respect is always better. At last and as far as I am concerned, the best way to make sure the bottle is safe enough is the 2 first ways proposed by Nathan001 which are cleaning the bottles with soap and hot water first and heating them in the oven. Two great days tips which should always be taken into account.