Shutter spped for flash

I have never bothered with flash very much, but having recently acquired a 5D I have been experimenting with my 420EX flash unit. I am wondering what is the usual practice with shutter speed when taking flash. I noticed that in AV mode the shutter stayed open long after the flash had fired in situations where the room light was quite low. I went to Custom settings on the 5D, and enabled the 1/200 sec. option which works in AV mode.
I know I can go to manual mode and select other shutter speeds, but I am wondering what is the best practice.
I realise the ISO setting, the aperture selected and the available room light all have to be taken into account.
I would be grateful for help and suggestions on this topic, as I will shortly be taking some pictures at a social function.

Shutter speed doesn't affect flash exposure, so set it to get however much ambient light you want.

Best practice would be what you're comfortable with and gets the job done...

I have taken advice from forum members and turned on the safety shift function in my 20D rather than the fixed shutter speed (Cfn16-1). Works great in Av and Tv. Usually I shoot in manual on the camera when using flash - meter for the ambient light/background keeping the max shutter speed in mind, and let the flash take care of the foreground along with some possible FEC depending on the situation. Under controlled light, stationary people, I use 1/125 on the shutter.

bieber wrote in post #3474091
Shutter speed doesn't affect flash exposure, so set it to get however much ambient light you want.

Of course, that's a very over simplified statement but as long as we're talking about shutter speeds at or below max sync speed, that is a generally true for most flash units.

PacAce wrote in post #3474257
Of course, that's a very over simplified statement

My specialty But yes, I should have mentioned that this only works if your shutter speed is slower than or at your camera's max sync speed. If it goes over, you'll have to enable high-speed-sync, which makes you lose a lot of power.

The understanding that shutter speed affects the ambient exposure but not the flash exposure is a powerful piece of knowledge. Shutter speed is what gives you creative control to expose the background ambient as you see fit.

Conversely, you can use 1/200 shutter speed to basically eliminate the ambient light indoors, and when your flash is the only light source you can all but eliminate motion blur problems and color temperature issues. Skillfull use of bounced flash is normally required to get pleasing background exposure when you do this.