When you are taking a photo of an object, you need to try and centre the item in the frame. This gives the best photograph as it makes the image easier on the eye and a bit more symmetrical.
This is also a good idea as if you ever want to print the photograph out in the future, there is always the chance that some cropping may need to be done, especially if it is a strange print size you require. If the photograph has the object right up against one side, half of it can be lost when printing due to the cropping.
So always bear this in mind when taking photographs of single objects!