I do balanced fill flash in CW manual, ap or sp all the time. No matrix meter needed.... Just expose for the scene correctly and set you flash up correctly and your set. Make sure you don't exceed the max flash sync speed when you set the shutter speed.
Nikon F4 in manual exposure mode CW metering 80-200mm f/2.8 with SB-800 in TTL BL. Film E100G
Matrix balanced metering though allows me to shoot in a wider range of situations because the camera meter is "smarter" than the flash gun's meter. It also requires less thought so I can devote more thought to composition.
Mark Barendt, Beaverton, OR
"We do not see things the way they are. We see things the way we are." Ana´s Nin
Matrix metering is a great generic metering tool that will hit just the point in many situations. Of course matrix metering provides you with what camera things correct and not what you always have in mind. This is why CW metering becomes more useful after time, when one becomes more demanding and develops an eye for certain details in the frame.
When the lighting is even, matrix metering can never go wrong, but in times of high contrast, say noon time, you have to tell to the camera what you are aiming for (highlights/shadows).
You can either learn the behavior and tendencies of the matrix metering of your camera and compensate the exposure when you thing it won't give you what you want, or stick with CW and always point to it what you want to expose for.