In my non-disclosure agreements, I include a clause to the effect that information shall be deemed confidential whether or not it is identified as such at the time of disclosure . In other words, information is presumed confidential unless otherwise noted. That does not mean that a discloser can be casual. I further advise that you use that rubber stamp until you get tennis elbow. Rarely is information purely confidential or purely non-confidential.
Most information is a hybrid of the two.
My advice is simple. In pushing for a marking requirement, the recipient will probably argue that he is entitled to know, at the time of receipt, exactly what information the discloser considers confidential. Otherwise, the recipient will not know how to treat the information that he receives. On its face, that sounds reasonable enough. In practice, it is anything but reasonable.