Technically, if you don't wear glasses when you need, it will make your eyesight worse. If you want to see something clearly but with poor vision, you will try hard to strain your eyes to focus the target. If you often do that, it makes your vision more worse. If you don't want to wear glasses, you just wear it when you need. In your daily life, you can take off eyeglasses if you it won't affect your life.
That depends on your own condition. If you are slightly nearsighted and can see most things clearly without glasses or contacts, then your eyesight won't get worse even if you don't wear glasses or contacts. However, if you are highly nearsighted, your vision might get worse without glasses or contacts. Because you can't see most things clearly without glasses or contacts if you have strong prescription and you always get blurry vision. At the same time, you try to strain your eyes in order to see them clearly. Thus long-time eye strain will make your eyesight even worse.