Even the best-laid plans can change once you have the weight in your hands and you're already tired. You may plan to take off just 10 pounds on a dropset, but find that, when the time comes, you really need to pull off 15. That's OK. When performing these dropsets, make sure you communicate that to your partner. If you're the spotter, pay attention to your partner and watch for indications that he may want to make a change.
Drop the weight when you're unable to complete the reps with good form, but otherwise keep the weight as heavy as you can manage. These dropsets are engaging muscle fibers you couldn't have reached without placing yourself in a constant state of exhaustion, so it's important to challenge yourself by pushing out those last 4 reps.