The reasons as to why proper technique is essential are clear:
- Minimize your risk of injury.
- Optimally target the muscle groups you want to work.
It is especially important when doing a new exercise that you learn how to do it properly and stop at the point of failure (when your technique begins to break down during a set). As you are doing an exercise, you are cognitively hardwiring in your brain how it should be done. If you are doing an exercise with poor technique or continuing past the point of failure, you are laying down the hardwiring in your brain how to do an exercise badly.
It is harder to relearn how to do an exercise with proper technique than it is to learn it from the start. This factor is most relevant for the big lifts such as squats, deadlifts, cleans, etc. because they involve the whole body. It is easy to let the bar get the better of your weak muscles and pull you out of the correct posture.
If you are tired or not feeling 100% up for it then you are probably best to back off on the big lifts and save yourself for next time.
Once I set in place the hardwiring of how an exercise is done, using proprioception, I find it easier to push out the last few reps of a set without losing posture.