Of course Nanna is right.
Although for a single SW defects in the center of a 10times long nanoribbon both solutions are fine (optimize bulk conf. and create device - create device and optimize device conf.)
Note that in the electrode extension regions of a device configuration the positions of the atoms are kept fixed and they match the atom positions of the electrodes.
http://www.quantumwise.com/documents/tutorials/latest/ATKTutorialDevice/index.html/chap.input.html#sect1.input.deviceIn this respect, if you break the periodicity in a region close to the walls of the cell during the optimization of the bulk configuration, the "Device from Bulk" plugin will not recognize the periodicity that you need to create the electrodes.
Hence, you will have to do it manually.
(Actually, it may recognize the periodicity of a "distorted" structure.)
So, "If you optimize the ribbon as a bulk system, you MAY HAVE to insure this manually" always applies.
This should answer to your second question.
About the first question the answer is: no, if the plugin managed to do it automatically.
It is hard to give general and universal answers, you may need to adapt from case to case.
But I hope everything is more clear now, is it?