This varies from person to person. I always like to do at least 1 or 2 forced reps with a lot of my sets only because I dont have a spotter at home and when I train at the gym it really makes my workouts intense. Using them excessively will quickly lead to overtraining so I wouldnt use them consistently.
An idea that I have used that has worked well is this. I have literally taken all of the intensity techniques that I know of, drawn one from a hat, and used it for a week or two. It may sound crazy but it really keeps the body guessing big time. Ill do forced reps on all my exercises for a week or two, Ill do drop sets at the end of each bodypart for a week or two, 10x10s, negatives, static holds, you name it. Just a suggestion. Good luck.