Using Flash is very Tricky. Flash is more than just an illuminating tool.
I hope TS knows that when on flash... and presuming he sets his flash unit to ETTL (Canon I suppose) and that his camera is set to Manual Mode.. He has full control of the Camera. Flashes can over power the SUN and available light at times.
Presuming that it is already known that Shutter Controls Ambient Light and Aperture controls Subject Light. Small Aperture on ETTL will make your flash work harder.
As for WB... I think there is like... no set rule... but the end result must be pleasing to your eyes and that you need to take note of the colour of the ambient light. I hope TS has a sensitive eye for colors.
Taking that all flashes 'emulate' the suns colour/ temperature.... which is about 5500~5700~5900K
Varying situations I can Think of...
Strong warm Spotlight indoor.... I would set to WB to Tungsten. To Balance things up.
Turquoise wall and a Chinese Lady, Both look flat indoors, .. I would set the WB to Tungsten (preserve the cool of the turquoise wall) Put a Warm Orange/ Tungsten gel , use an offshoe flash cord and do a wide direct flash on my subject.
I set another Situation where I read of Joe Macnally.. when he was shooting with Film...
" To match the fluorescence lights (in the subway), I put on a green gel on the strobe, a 30 Magenta filter on the lens to clean up the fluorescence and give me a good skin tones... (Set at Daylight WB)..."
Really.. to get the desire skin tone, you will need to look at the colors that is bouncing off everywhere indoors and match it. Then you adjust accordingly... Of course... YOU MUST HAVE THE LUXURY OF TIME.. like a few seconds to adjust. Takes time to be perfect... but more importantly when you look at the LCD (another Luxury).. do you know why your colours turned out that way???
Take a Flash photography course maybe to understand your flash better...and keep practicing. I prefer to get it right in the camera. If all fails.. there is always.. P.Shop....:bsmilie:
Cheers!