Amfibius' point is taken. But choice of words unnecessarily provocative.
As a RULE, use a lens hood, both for reducing flare and for lens protection.
However there are situations as mentioned by someone else where not using any lens hood do not lead to image degradation. An example is shooting in a flat light with the light behind the photographer.
I shoot with 35 mm, medium format, and large format. For the 35 mm and medium format, I almost always have the hood on, except for situations where I forget to one on. In situations of harsh lighting, especially when the light comes almost from the front, even the manufacturer's hoods are often not up to the task, and I resort to even bigger hoods such as those made by Lee and Cokin, making sure there is no vignetting.
With the large format when everything is slower, using a compendium hood such as the Lee's, it can be a hassle to put on the hood. Under these circumstances, I very often shoot without a hood, but first looking at the lens front element to make sure stray light is not falling on the lens. If I have any doubt, out comes the Lee compendium hood!