From a muscle standpoint, yes, it can be harmful. There's a technique called Alexander Technique which focuses on re-educating the muscles to do what they were meant to do. When it comes to posture, we often times use superficial muscles to do involuntary tasks - in other words, we use all the wrong musles to do things that our bodies are capable of doing on their own.
For example, posture - the skeletal system is designed to support and balance itself. Therefore, it takes little muscular effort to have good posture. When a person tells you to "stand up straight", you'll notice that you puff your chest out, arch your back, and pull the shoulders upward, etc. As a result, you'll feel tension in the neck, shoulders, chest, back and abdomen. Because extra muscles are being used to complete an involuntary task, you're putting excess pressure on nerve endings and joints which feeds into the perception of pain, shallow breathing, and potential damage to joints.
I use the Alexander Technique often with my patients in conjunction with Progressive Muscle Relaxation to address issues with posture and pain. It's amazing to see how much more efficiently the body works, and how much less aches and pains are felt, when we don't over-do it.