Are the Titles of the Books of Scripture Canonical?

One of the questions I sometimes get from students in the Seminary is this: Are the titles of the books of the New Testament also canonical? Or only the contents?

      Along these lines, I've recently reading Bruce Metzger's classic, The Canon of the New Testament: Its Origin, Development, and Significance (Oxford University Press, 1987). In his chapter on the closing of the canon in the West, he makes two fascinating points about early questioning of ancient book attributions by Catholic scholars. First, he points out that many of the doubts about apostolic authorship of certain books of the New Testament can be found already in the work of Cardinal Thomas Cajetan, O. P. (d. 1534). 

                                                       Cardinal Thomas Cajetan, O. P.

Unfortunately, Metzger does not give any direct quotes from Cajetan, but only points out that he  denied Pauline authorship of Hebrews and expressed "doubts" concerning James, Jude, and 2 and 3 John. 

     Second, he also notes that the the famous humanist, philosopher, theologian and Catholic priest, Desiderius Erasmus (d. 1536), likewise denied the apostolic the apostolic authorship of Hebrews and James and raised doubts about 2 Peter, 2 and 3 John and Jude. 

                                                              Erasmus of Rottersdam

What surprised me most was this: despite Erasmus' reputation for being a 'free-thinking' humanist, in the wake of criticisms for raising doubts about the authorship of various books of the New Testament, he apparently responded as follows: "If the Church were to declare the titles they bear to be as canonical as their contents, then I would condemn my doubts, for the opinion formulated by the Church has more value in my eyes than human reasons, whatever they may be." (Erasmus of Rottersdam; cited in Metzger, 1987, p. 241).