Depends. The goal is to work the muscle to failure. Using a heavy weight a few times doesn't engage all the muscle fibers before the initial ones tire out.
Since I'm not interested in building mass, but basic strength and endurance, I like to use a weight that is about 1/2 of my goal weight to warm up with a couple of sets of 15-30, then one set of the goal weight.
Since it is good to mix things up, I also like to sometimes go low weights, high repetitions (20-50/set) for a few weeks, or a few weeks of high weight/low reps (5-8).
Increasingly, I find myself using body weight type exercises, with a step ladder of repetitions to work to failure. If I do that with push-ups until it becomes a strain to do 15 push-ups, then the next day I'll feel like a whipped puppy.
Now for the real question - why do I work to feel like a whipped puppy?
No pain, no gain!
Because you still feel guilty about being a teenage bedwetter? ;-)