Don't sacrifice your form to lift heavier weights. You'll make more progress BB curling 70 pounds correctly (without swinging your back or moving your elbows) than by curling 90 pounds. You just look like stupid when you are lifting weights that you cannot handle. What’s the point of going to the gym? (Vainly increasing numbers, or actually making progress and being able to handle those numbers). You can sacrifice some form every now and then (My form for BB rows is pretty bad sometimes), but don’t do anything major or you will end up hurting yourself.