Which white balance setting on my digital camera is best to use when shooting in sunny weather with fill-flash?
These types of questions are always subjective. People in internet forums and camera clubs will gladly offer advice, but often it's nothing more than pure speculation, and it assumes that your taste in photos and your photographic objectives match those of the advisor. Most of the time, you'll get conflicting answers and you'll still be none the wiser!
Any time you feel the urge to ask a bunch of people "Which setting is best to use in this situation?", don't. The only way to get a meaningful answer is to perform your own controlled test and use the results to make your own decision as to what you think looks best.
For the White Balance example above, get a friend to go out into the sun with you and shoot some test shots using flash. Try all your white balance settings - Auto, Custom, Sunny, Cloudy, Incandescent, Fluorescent, etc. Also vary the output of the flash you're using - fill flash is often one or two stops below ambient in bright light situations, so do your test with -3 stops, -2 stops, -1 stop, 0 stops, +1 stop, +2 stops and no flash. Try putting the person in full sun, half sun and shadow. Make sure you take notes so you can work out which shot is which later!
You now have a comprehensive set of images to study. Go through them and see which ones you like. Are you going for accurate colours, such as you'd need for fashion or product shots? Or are you going for rich, warm colours for beautiful portraits? Or are you after a different look altogether?
Since we bracketed the flash power, you can also see what setting gives the fill-flash look you're after. Do you want deep shadows or just a hint of shadow? Should the flash be noticeable or discreet in the final image?
The choice is yours!
If you've done the test and still aren't sure which combination of settings looks best, go ahead and ask others. You'll get much more useful feedback if you post a few sample photos in an online forum or take some prints along to camera club and ask some experienced people which shots they prefer and why.
One way to speed up this test shoot is to take all your photographs in RAW mode, if your camera offers it. You can then play with the white balance settings when you convert from RAW to jpeg on your computer later.
Whatever setting you feel works best, write it down so you won't forget. I like to keep a little notebook in my camera bag summarising the results of these types of tests so I don't have to work it all out again in the field.