When reading Bhai Sahib Randhir Singh jee's life, we see that when he was doing Siri Rehraas Sahib and his officer called him, Bhai Sahib did not stop his Nitnem Paath to answer him. He kept callling but Bhai Sahib stayed engrossed in Paath. Later Bhai Sahib told him that if he disturbed him again, he would quit his job. This Saakhi proves that while doing Nitnem one should not pause and continue till completion.
On the other hand, we see that when listening to Siri Sukhmani Sahib from a Singh who accidently cut his finger, Bhai Sahib upon realizing this, interrupted him once he had completed the Pauri or Ashtpadi. This proves that non-Nitnemi Baani can be stopped, in case of the need, after completing the current Pauri or Ashtpadi.
Guru Sahib knows better.