I've find a cool way to use safety shift in 5D. If you use Tv mode and safety shift (ISO), camera will use as high ISO as needed. It actually works identically w/wo flash.
The best thing is that you can use autoISO or set ISO manually. If you set ISO manually, consider your setting as a minimum ISO setting, camera will rise it anyways when needed. This is very nice feature in low light, where fill flash is needed. You can, for example, set high ISO to have f2-2.8, but when it gets darker, f1.2-1.8 are used and if that is not enough, ISO will be risen even more.