Muggles' Guide to Harry Potter/Magic/Grim

Overview
The Grim, an omen of imminent death, has the appearance of an extremely large black dog. Being spectral, it is visible only to the person whose death is imminent.

Extended Description
At a number of points in Harry Potter and the Prisoner of Azkaban, Harry sees a large black dog; the first two times he sees it, he almost immediately narrowly escapes being killed: the first time, he is almost run over by the Knight Bus; the second time, debilitated by Dementors, he falls fifty feet from his broom; in neither case does anyone else see the dog, and at a second look it has vanished. In between, he has found, at Flourish and Blotts, a book of death omens with a picture on the cover that matches his recollection of the dog, and Professor Trelawney has read his tea leaves in Divination class and seen a Grim there. Later, Professor Trelawney apparently sees a Grim in Harry's crystal ball as well.

Taken all together, this leads Harry to rather believe in omens. However, he does see the big black dog again just before the Quidditch match with Slytherin, and that time it appears that Crookshanks can see him, so he begins to wonder if perhaps the dog is merely a dog, rather than an omen.

Questions

 * 1) Do you believe in omens such as the grim?
 * 2) How superstitious is the Wizarding World?