When trying to increase the amount of load on a given lift many people try to max out every single session or go to a near max every session. This is a common mistake that more often than not leads to you hitting a plateau. This could be due to burning out of the nervous system or a change in lifting technique, among many other things. I mention these two as it is what I see most regularly. The change in technique is a bigger factor for most of the people I coach or see in the gym.
I am not against grinding out reps here and there. I think it is something you do need to learn how to do but it shouldn't be something you do every single set of every single workout. When grinding through a sticking point your technique should not change. If you take the bench press as an example, you will see people tilting the bar from side to side, worming around underneath the bar, or kicking their legs out in air. This will ruin your lift, not improve it. Strength training is a skill. You are teaching the body what movement pattern you want to produce each lift. Don't ingrain poor habits and poor recruitment patterns by being sloppy trying to lift a load that is currently too heavy for you. You will also dramatically increase your risk of injury.
Each rep of the set should look exactly the same. This is something I learnt from Charles Poliquin a long time ago. So if your first rep of the set was perfect technique then your last rep should look the same. Sure the bar speed may slow down a bit but you should not have to change your technique to lift the load. If it is the back squat, and you start with your torso fairly upright and use good leg drive out of the bottom on the first rep but then towards the end of the set start shooting your arse up first and start to stripper the load then it is time to call that set.
There is a definite benefit to taking a lighter load and making sure you smoke all reps in the set with solid technique. It will improve your bar speed and help re-enforce good technique. Your ego may take a hit for the first few weeks but you will surpass where you could have gotten to by just grinding away each day. Also, it isn't a huge drop in weight. I'm not talking about going to 50 or 60% of maximum or anything like that. For some people it may be as little as 5-10% less than they are currently doing. I don't use percentages in training so it is just a load where I can keep perfect technique. If bar speed slows down from one rep to the next and if going for another I feel like I may need to change technique to get the rep, I call the set there. Probably leaving 1 rep in the tank, if not 2.
Here is Marcus Moore back squatting 97.5 kg x 3 reps. He had followed a cycle where he was not allowed to grind reps, as in the previous cycle I felt he had a few too many. He started this cycle at 92.5 kg x 2 for 5 sets and by the end of the 4 weeks took it to 97.5 kg x 3 for 5 sets. It is not a PB but it has set him up to smash his old best in the next couple of weeks.