Some additional information to help you understand *why* people are rx'ng sets of 8:
"Assistance work" really falls into two categories. One category is partial range of motion exercises like rack pulls. These are "assistance" because you're doing them to help you with the main lift (and hopefully not for bragging rights due to the fact that you can lift more weight through the shorter distance). But they are often done for sets of five.
The other category of assistance work is exercises that isolate specific muscles. These are usually done in sets of higher reps for at least two reasons: (a) you're doing them for aesthetic purposes anyway, and higher reps builds bigger (but not necessarily stronger) muscle, and (b) it's difficult, due to the nature of these exercises, to do them at closer to your limit weight without excessive cheating, which negates the benefit of isolating the muscle.
There appear to be two schools of thought in the weightlifting world. Some people express disdain for isolation exercises and insist that multiple joint lifts are the only thing that you should be doing. On the other hand, if you go to any of the websites with Westside programs, you'll see plenty of face pulls, lateral raises, etc. etc. And those people are pretty strong. If you look at Wendler's e-book, he swears by dumbell rows in sets of 20 or more.
The contradiction is sort of an illusion, though, fostered by people like Mark Rippetoe, who are really primarily interested in weaning novices off of bodybuilding programs that aren't doing anything for them and introducing them to a core lifts program. If you read SS, his real message is that assistance will help. Later. Meanwhile, if you want biceps, learn to do chins.
In any event, doing curls between sets of your program just to have something to do is eventually going to interfere with your ability to complete the sets in your main lifts. But you will hopefully notice when this happens and correct.


