This is a huge mistake.
It's important to train your back not only to improve your overall appearance, but also for health reasons. It's imperative to work your back muscles to keep your shoulders strong and healthy.
Add the following back exercises to your workout routine two to three times per week. Focus on every rep and improve your performance when you repeat workouts. This means using more weight (or less assistance for assisted pull-ups) or performing more reps with the same weight.