You may not feel the same kind of burn during these low-rep workouts that you're accustomed to, but strength training inflicts plenty of stress on your body. Your muscles and nervous system are the victims of your training, and they need time to recover, adapt, and come back stronger. If you drag through the final workout of the week and see your reps and weights go down, you aren't resting enough—it's that simple.
Don't cheat here. Rest up now, and save your enthusiasm for tomorrow, when you'll need it.