I own Metz AF-2 (works fully auto and manual) and Yongnao YN-560 (manual only) and have used them both at weddings. I try to do without flash for as long as I can and once I do decide to use flash, I pay a lot attention to balancing ambient light with flash power - something I still need to master though.
If you own f2.8 lenses then I suggest you do the same. You should get good results up to ISO 3200 and even 6400, but you can of cause add in the flash if you do not want that high ISO.
I only shoot in manual (cam and flash). This allows my exposures to be more consistent, but you do need to pay attention to exposures a lot - I'd say you need to do that anyways though.
I also always (when I can) bounce my flash. When indoors in somewhat normal/bright conditions I'd be shooting with something like ISO 800-1600 @ 1/125th or faster. Flash power depends on aperture being used.
When it gets darker as in late evening/night, I drop my shutter speed to something like 1/60th and boost the flash a bit.
Pay attention to subject distance. The closer they are the more light they will receive.
Try PTTL and manual during day and evening before the wedding so you can get even the slightest feel for it - try in larger rooms. Small rooms will make it looks like a piece of cake thing and it's not.
If you plan on shooting outdoors against bright sky, such as during sunset then a flash unit that supports HSS is a must and the YN-560 is no longer an option for you.
Tip 1: You can rotate the flash head to control the power - this is often faster than operating a flash. The YN-560 is very quick to operate though.
Tip 2: Use whatever fits the criteria to block the flash head to cast light directly at your subjects - this reduces reflections on skin, glasses etc etc. a lot and the end result is
awesome:
the black foamie thing ..
pro vs. typical flash usage.
Tip 3: Get some gels - you'd probably want to use orange gels (tungsten light is most common, but there are no guarantees) on the flash head. It will help balance the color of the flash light with the tungsten light at the location. This is often overlooked but the end results look much better than using no gels since the temperatures/color of your flash and the majority of lights at the location will be closer together.
Shoot RAW get extra SD cards if you need to.