Well you can use whatever you want that gives the right exposure you want. Find out just how long your flash will flash at full power. My Metz is 1/300 at full power. If I use 1/400 the shutter speed is shorter then the flash duration. Not good.
1) For fill flash outdoors meter the scene and use that F/stop and shutter speed. Just set the flash head at one or two f/stops wider.
2) Indoors if you want no ambient light then use a faster speed.
3) If you want more ambient then use a slower one. Or if you want the flash to light the foreground and the ambeinet the background then meter the background shutter speed.
If you basically just want to use the flash indoors then set it at something slower/equal to your max flash duration.
Hopefully I didn't make too many mistakes.