I would use the U lock on it when its parked outside for a quick run like coffee or something. Depending on how safe you feel it is, freelocking it might be a pretty good deterrant, as no one can ride off with it - the most they could do is try to take a wheel (assuming the lock isn't through one of them/both of them) or try and throw it in the back of a truck.

I too lock my bike as habit. Better safe than, "Oh s***! Where the **** is my bike!"