A model release is needed from each person whose likeness appears in a photograph that is used for advertising or trade (business) purposes (such as selling the image on prints) when the person is identifiable. … If the answer to that is No, then you do not need a release.
However, you may not know at the time if you will be using the image for such a use. So I always err on the side of caution and say you should get one when you can.
Once you press submit on this form, a ready made release form will be sent to your email. You just have to print it out and keep handy for those times you need one.