Back-off reps necessary?

[vigilance]

Hi all,

I haven't been able to find anything in the Blue Book about back-off reps. Is there an "official" stance on this?
Throughout my NLP so far there have been several times when I felt I would have benefited from doing these.
According to Rip, if you fail to complete the 3x5 prescribed reps for a lift you repeat the same weight the next time,
but wouldn't you have to do the reps you missed anyway, during the current workout, so as not to miss out on the needed volume?

Thanks!

[Mark Rippetoe]

Practical Programming for Strength Training, 3rd edition – The Aasgaard Company