I'm still considered a newbie but I believe I was told to wait until you have 8-10 inches before you cut. I guess it can die when you cut it and it will travel back up towards the bananas and sometimes the bottome bunches end up being affected and dying too...I think???? I don't usually answer ?'s because I'm NO EXPERT but I remember waiting to cut mine until it was 12 inches and I'm glad I did becasue it took a FULL 6 months for those naners and the bottom of the rachis did shrivel and die up a little. Anyway, I feel very cool answering a "technical" banana question.
I hope I answered it correctly.
Jen