I think Canon's ETTL2 flash system is a bit of a mystery to many, certainly me, so I've been trying to learn a bit more about how to safeguard my shots. Here are some tips...
In Av or Tv mode the flash will assume it is providing fill and will set an aperture or shutter speed to capture the ambient light correctly or underexpose it slightly. It will then top up as it sees fit (by default probably not enough). This can lead to some surprisingly slow shutter speeds. Take control and shoot with manual exposure for the background.
Coupling the camera with a lens that passes distance (focus) information and the flash head in the straight ahead position ETTL-2 will use that distance information to feed into the exposure calculations. If you put a modifier on your flash (Omnibounce, for example) the camera will have no idea that you have fitted it and the light emitted will be calculated based on a direct line of sight to the subject. Your shot will likely be underexposed.
ETTL-2 likes to protect highlights at all costs. Anything reflective or very pale that gets caught in the frame - glass/metal/white paint/wedding dress - will cause the flash to quench early. Be ready to add a fair handful of +ve FEC in these situations. If you know a white wedding dress is your primary subject then you will have to turn up the FEC to get a white dress instead of grey.
Here is a series of test shots to show the effects of ever increasing FEC from +0 to +2 stops in 1/3 stop increments. I mocked up a white dress by using a white sheet and shot in my somewhat dim hallway, with a hint of backlight from the windows behind. I metered the ambient and set exposure manually to be about right and then let the flash take care of the subject. The histogram shown is for the last shot, taken at +2 FEC. I think that is the only shot that would even begin to do justice to a wedding dress. Whatever you do, double check your histogram on the critical shots.
Good luck