There's the other side to this coin, and that it can actually work against you. As you get closer to completing your assigned reps, fatigue may set in quicker based more on psychological factors influencing the physical ones. An alternative (not better, just an alternative) would be to have someone count your reps and only reveal them to you when you have become truly exhausted physically.
I found when I employed that tactic, it served to spur me on and generate some more energy from a psychological point of view, feeding into the physical instead of robbing it. An example would be feeling good and wanting to go high reps (the way I did with those 120kg x50). You just crank up as many reps as you possibly can before running out of gas, then (and only then) you ask for the count to be revealed. If the man tells you (say) 23, that would be enough to regenerate whatever reserve you have to stretch it all the way out to 50 if possible. It's just another way and another side of the coin.
After the count has been revealed, then Andy's tactic would serve in a truly positive way, as you'd really want to add 1 more rep at a time to get the maximum out of yourself.