It depends on the obstacle, the weather, whether you're using a fisheye lens or a telephoto lens or neither, the lighting etc

Your best bet is to look at any of the photos on this site and read the comments. If someone has said that the angle's bad, look at the picture and try and understand why the angle is bad; the same applies for good photos

General rule of thumb for fisheye shots is to get low and angle the camera up a bit. And never shoot into the sun unless the silhouette is well defined, otherwise it'll just look shit