Your tire will not lose air and deflate without a valve cap, but If you do lose a valve cap it is better to replace it than to leave it. If your valve is leaking, then the valve cap will stop air from going out. Another important role of the cap is keeping dust and debris from entering the valve stem and compromising the valve seal.
Everytime you inflate your tires the dust can get stuck on the seal compromising it and air will start to leak out.
