When strength training, it may seem that if you use a heavier set of dumbbells, you can save time on your sessions. More weight means fewer reps, right? Well, not exactly. Form is more important than how much you can lift. So if you're struggling with 10-pound weights when doing your bicep curls, you'll end up hunching your back and pressing your hips forward, which could result in a pulled muscle or other injury. Also, correct form will ensure that you're targeting the specific muscle you're trying to work in each move.
A good rule of thumb is to choose a weight that tires your muscles out after the 10th or 12th rep. By the end of your set, you should still be able to maintain correct form. If you can't, do fewer reps or choose a slightly lighter weight.