DO NOT METER off the face if it's very fair or very dark. Your exposure will be out.
Metering itself is a complicated subject, and being in the middle of a wedding doesn't help either.
The first thing you must absolutely get right is how your camera meters in DIFFERENT modes, and know when to use them. Partial, evaluative and center-weighted will all give different results when there is an off-centre light source (eg light or window). When you're moving around, eg tea ceremony, these light sources change in size relative to the frame and in intensity. Unless you're in manual mode, the compensation dial will have to keep turning to adjust to the frame. Takes a bit of experience (read: trial and error) to know how much to dial and when.
Another alternative is to use a grey card and fix the camera in manual mode. And stay alert for lighting changes.
Do a search on 'weddings' here. i think you'll turn up quite a lot of reading material, on both exposure and flash exposure for Canon systems.