Tips » Weight Training
I'm happy with this muscle or that muscle, so is it okay to leave it out of my training program?
Keep in mind that you should always balance your training, not necessarily for appearance, but health and injury prevention. For example, if your calves are fine, fine. If you are working your quads, however, you should work your hamstrings and glutes. If you don't, you'll create an imbalance with a stronger quad pulling on your hip joint without a hamstring to support it. This can lead to lower back pain and hip problems.
The same thing with big biceps. A lot of people focus on biceps. Unfortunately, the key to big biceps is triceps because triceps is 2/3 of the arm mass. You simply do not have the foundation to grow your biceps size without powerful opposing triceps muscles. Finally, overworking one side of the equation will simply lead to an imbalance around the elbow joint, and then you get into painful popping/tennis elbow, etc.
It's okay to prioritize a body part that is important, but neglecting opposing body parts is a recipe for injury. Also, sometimes the logical thing isn't always right. Case in point: let's say you want to increase your chest size. Training chest is important, but training back/rowing is very important. If you have a tight back, your chest muscle will be constrained and simply will not be able to grow. With a loose back and stronger back muscles/larger, this provides more opportunity for the chest to grow as well. It takes a holistic approach to improve your body!