For me, when i do wedding shoots for my friends, I would almost always use Av - Aperture Priority, unless special occasion arises. This is because i'm almost always doing portraits for weddings. I just love that creamy bokeh of my 70-200 F2.8 @ it's widest aperture when using it for portraits. Don't you? This focal length is perfect for all my portrait shots with my 5D. I had learnt to always watch out for my shutter speed (learnt it the hard way) for the focal length that i'm using to prevent handshake/movement blur. Use the exposure compensation to compensate for metering of scenes.
Having done several weddings, I've learnt that impromptu events happen all the time and fidgeting to get the exposure will almost garunteee that you'll lose that moment. I don't even change lens during a wedding shoot. Unless it's studio shooting where you'll have the luxury of time, full manual mode just dun make sense for me when shooting events (weddings proceedings are considered as events in my book).
Some things that i've learnt is that to always set the shutter mode to continuous shooting. When there's an important shot (die die must have shot). Squeeze off a few frames. You'll be surprise at the different expressions of the bride's/bridegroom's smile at different frames.