Are you installing it on a vertical surface? I am used to installing soffit under eves. The only part with wood "behind" is the J channel. Unless by "behind" you ment "above"... in that case then no. Water should never get up into it and even if it did the soffet should have enough venting to dry out easily.
That's what I thought, my father-in-law is managing the build and he some times gets overzealous. He wanted to put water & ice up, I said that it could trap moisture.
