The repetitions should dictate the weight you use. What this is implying is that you need to have a game plan going into the weight room. You should have your reps and sets determined ahead of time. I see too many guys at LA Fitness walk up to the barbell bench and put on 45lbs on each side and just start lifting. The problem is that they do this every time they go to the gym. Change up the reps, which will require you to change that weight you use. If your goal is to train for maximum strength, you will likely only do 1-5 reps that will require heavier weight than training for higher volume, which can be 12-20 reps. The idea is to have a program designed ahead of time so you do not blindly walk into the weight room and select any dumbbell off the rack and lift it until you can not do anymore. The weight you choose to lift will affect the stimulus your body receives.
Train Hard-Train Smart