Haamidaw wa Musalliyaa
It is Makruh to stand alone behind all the rows if there is place in the rows in front of one. On the other hand, if there are no gaps in front then it is not Makruh to stand in the Saff alone. However, the juristic text from Badaai-us-Sanaa'i mentions that it is more appropriate for one to wait for a person entering the Masjid so that one could commence the new row together with that person, rather than standing alone. However, if one fears that one would miss the Rak’ah, he could touch a person who has knowledge of this ruling and has good character to indicate to him that he should come back to join the newcomer in the new row.
This conditional stipulation is meant to avoid any problems that might arise due to lack of knowledge or understanding on the part of the person who would be so disturbed. If the newcomer is not aware of any such person in the row in front, then he should stand alone at the back, in line with the Imam.
And Allah Ta'ala Knows Best
Shafiq Jakhura (Mufti)