if you have high prescription which is more than 400 ,then you'd better wear them all the time . Occasional wearing will impair your vision .but if your prescription is low , you'd better wear them only when you need . Long-time wearing can cause your prescription go higher .
It is depends. If you mean you have perfect vision but wear prescription glasses, it will hurt your eyes and also may cause headaches etc.
But if you have poor vision and wearing prescription, you can choose to wear it no matter you need them or not. The eyeglasses will do nothing but provide you the better eyesight.
So, whether wearing glasses damages your eyesight or not depend on your eye power.