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1. If you are photographing people, understand the difference between key and fill light. The light source that is providing the dominant pattern on the subject is the key light, and the depth of the shadows is controlled by the fill light. Thus, if the ambient lighting (for example: sun outdoors or overheads indoors) is creating raccoon eyes, you can either use flash as fill, in which case the raccoon eyes are still there but are lighter, or as key, in which case the raccoon eyes are gone completely as they have been overpowered by the strobe. The latter case is sometimes inappropriately called "fill" flash, when the strobe is actually the key.
2. If you are photographing indoors in a dark room, the strobe is typically providing the key light, as in most examples posted in this thread. I seldom see true fill-flash used indoors, although its possible of course.
3. When bouncing the flash, the placement of the bounce surface is analogous to placement of a key light. In most circumstances, a light placed not much more than 45deg above the subject in a 180 arc in front of them makes the best key light. Thus, the corner of a wall and ceiling creates the most classic angle for a key light in terms of height in most normal rooms.
4. Bounce cards, sto-fens, etc.. effectively create two light sources. Depending on the distances and power involved, the card/modifier may become the key, or more ideally (in most situations) they will provide some on-axis fill while allowing the bounce to remain key. This additional fill may not be necessary in all lighting conditions.
5. In general, on-camera fill from direct flash or a bounce card is almost never beautiful. It may be useful, especially if no other options are available. It may enable an editorial statement in some special cases. However, it is almost always second choice to other methods if exquisite lighting is your primary concern.
6. Using a one-size-fits-all approach with bounce and modifiers (IE always placing flash at a given angle with a given modifier) will likewise not produce the most elegant results 99.9% of the time. It may be a functional choice under some circumstances. Understanding the properties of light and the classic techniques of portrait lighting, along with how they may be adapted, will allow you to make decisions based upon your particular goals and limitations in a given circumstance.