Most of the time you want to make your target muscles work as hard as they possibly can through a full range of motion because you're wanting to stimulate and overload as many muscle fibers as possible. More muscle fiber stimulation = more growth (this is why squats and deads are recommended for gaining mass). But there are exceptions to the rules, such as "cheating" and partial range of motion reps, among other things.
There can be a time where you can use "cheating" techniques to your advantage. And in my book, there is a difference between this and just simply having sloppy form. For instance, the push press is a cheating exercise for the shoulders. When you can't get any more strict reps, you use a little momentum (as little as needed) by driving with your legs to help hoist the bar overhead. Doing this allows your shoulder muscles to continue working past the point where you'd normally have to quit. I also tinker with cheat curls, which is essentially the same principle as before: using just enough momentum to curl the weight, making the biceps work as hard as possible (I still make sure to keep my negatives slow and controlled). But "cheating" is something you want to do to make the exercise harder, not easier. A lot of people simply trade heavier weights for sloppy form, thinking they're progressing when they really aren't. If you add more weight and get sloppy, you may be able to lift more, but your target muscles didn't suddenly get stronger; you're just using other muscles to help lift the weight.
If you're trying to build a quality physique, you should be more concerned about how effectively you're training the muscle, not about how much weight you're able to throw around. So other than the occasional incorporation of cheating movements into your routine, there's really no excuse for sloppy form. Now you'll inevitably fall out of strict form with an exercise. Say you're doing heavy barbell rows. Eventually, usually towards the end of the exercise, you'll start swaying a little more to help get the weight up. I think this is perfectly natural and okay when doing heavy compound movements, but should be kept to a minimum. And there's very few reasons to ever let this happen if you're doing a specific isolation exercise, because strictness is very, very important when doing isolation work. If you catch yourself swaying on every set, you're simply using too much weight.
Strict form ensures you're targeting the muscles you want to work. You're making those muscles do as much work as possible, stimulating them as much as possible. If your form gets sloppy, then other muscles are coming into play. Like I said, there's a time and place for switching up your form with an exercise (i.e., cheating, partial reps, ect..), but there is no excuse for just having sloppy form. So my advice is, instead of worrying about how much you can lift, worry about how much you can lift effectively.
