You can over stimulate your bronchial tubes, they will feel sore, and with repeated use weaken. Most inhalers are steroid based, there are many side effects to consider.
While this will depend on the individual, several side effects can occur. Since many inhalers consist of a steroid of some type, the individual may have rapid heartbeat and become "hyper". In addition to this, it may affect the ability to sleep and cause headaches in some. These are only a few possible effects when the inhaler isn't needed.
Using an inhaler when you don't need one will probably not do anything to you unless you have high blood pressure and are prone to its' affect. You can't open an open airway more.
It would depend on the type of medication it contains. An inhaler is the apparatus used to deliver medications in the form of powder or aerosol into the lungs. In general, there are two groups of medications delivered - bronchodilators, steroids or in combination. So using an inhaler when not required would only lead to its potential side effects.