I have been told by my experts here at RCACF club to break in the batteries. It is just a longevity issue. With the prices of batteries as they are, longevity is an issue.
I have two 4S packs that I never broke in but they puff up very quickly.
My 5S packs have yet to puff up but I am sure they will hopefully not as much as my 4S packs..

Orlando