Strictly speaking, singing along with the notes will not help your fingering. From my experience, what I think the book is trying to teach you is to try and develop your ear. When you sing with the note, listen whether you're matching the pitch your guitar's making with your voice.

It's more about music theory than guitar techinque. By doing as the book says, you'll be teaching yourself (and your ear) about intervals and learning the difference between a half-tone and whole-tone.

My advice would be that if singing along with the notes is affecting your ability to play the scale, put it to one side for the time being and come back to it later. Long term it's a skill well worth having; however, you shouldn't let it stop you from progressing in terms of guitar technique.

If you do want to sing along with the notes, it's entirely up to you what you choose to sing. A-B-C-etc. is fine, or you could sing the DOH-RAY-ME-etc scales, or even LA-LA-LA - the important thing is to listen to the note coming from you guitar and match it with your voice. Whatever works best for you is the correct way of doing it...

Hope that's of some use...