I do not think a Catholic is free to dissent from Leo XIII's teaching in Octobri Mense (1891):

With equal truth may it be also affirmed that, by the will of God, Mary is the intermediary through whom is distributed unto us this immense treasure of mercies gathered by God, for mercy and truth were created by Jesus Christ. Thus as no man goeth to the Father but by the Son, so no man goeth to Christ but by His Mother. (4)

In fact, that translation (which I took from the Vatican website) is pretty bad. The Latin original is stronger, containing the words: nihil nobis, nisi per Mariam, Deo sic volente, impertiri. ("God thus willing, nothing is granted to us except through Mary").

I believe this amounts to an affirmation of the content of the teaching that Mary is the Mediatrix of All Graces, so I would disagree with the poster who said it is "not a doctrine." In fact, that encyclical essentially sparked the movement to get the doctrine and title dogmatically defined. Is a Catholic free to deny the teaching? Probably... but in my mind it is comparable to denying the Immaculate Conception before the definition.