If you bounce off the ceiling, a persons head will shadow the eyes. that tiny card will help to mitigate this effect.
When bouncing, I point the flash head towards the direction where I want the light to come from, essentially creating a large light source in that area. Especially when bouncing from an area behind the photographer, to avoid making the light too soft, it's a good idea to use the maximum focal length setting on the flash head since the light will bounce around in the room providing enough soft fill. If I want the softest fill I point the flash head behind me and slightly towards the ceiling. This may be too soft for a portrait, but it has the advantage that the light will go everywhere, filling shadows, so it works well for impromptu group shots. To get more directionality, I can point the flash to the side or towards a nearby wall. I use some black cellular rubber on the flash head to avoid the flash head being directly visible to the subject, which would create some wrinkle-accentuating (and potentially glare-introducing) hard light. The black material makes sure that all the light that hits the subject is coming from a reflection, leading to the feel of large light sources being used, much as they might be in the studio. It's possible to even create Rembrandt style lighting using an on camera flash bounced from a wall/ceiling with this kind of a black flag. However, it takes some iteration to get it just right.
I typically use flash for portraits mainly, for technical close-ups of small objects, and occasionally for documentary photos of people. I find that flash can be quite intrusive and change the subjects' behavior towards the camera and start posing for it, so I tend to avoid it in order to make a more genuine document of human behavior and social interaction. Sometimes it's necessary, however, to provide some fill light in order to avoid a harsh apperance of the subject. For example on a sailing trip in order to provide some fill light when the subjects were lit by the setting sun, I bounced light from the main sail and with some trial and error I could get enough of it to hit the subjects to create beautiful fill (basically M1/1). I dislike the look of direct flash fill as the natural gradation of brightness in the shadows is lost and skin blemishes are accentuated in the shadows. So I don't do that even in dire emergency. But the use of light is much as everything in photography: a matter of personal judgment and taste.
