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