MB 6: The portion of the everyday sacrifice - The Magen Avraham wrote that one who examines the Bais Yosef will see that there are various opinions why it is permissible to say these things from memory. Therefore one should be careful not to say anything from memory that is not mentioned here. The Sh"A [Shulchan Aruch, our Code of Jewish Law, is the logical expansion of this acronym... but where? -- YM] and the Responsa of the Chovas Yair permit reciting the entire book of Psalms from memory, for since they are being said to arouse Heavenly mercy, they are like prayer - and it seems to me that one can rely on this, for even without this, the opinion of the Ateres Zekainim and the Gr"a is like those authorities who say that recitation from memory is only forbidden when fulfilling the obligation of others.
MB 7: It is permitted - When one is fulfilling the obligation of others with his recitation of these writings, the Magen Avraham leans towards stringency, suggesting that one not say this things from memory in any event - and the same is apparent from the words of the Gr"a in his explanations.