For indoor ceremonies, first I determine if flash will even be allowed. Then I determine if I'll be close enough for it to make difference. Then I ask the couple's preference (I have examples of both flash, and no-flash during the ceremony in my portfolio). If it's okay to use I mix it up with some flash, some available light.
If the ceremony is outdoors on a sunny day, and I can get close enough, I use flash for fill. If it's overcast it just depends on how the couples eye sockets look. Some folks have deep set eyes, and they need fill flash even with diffuse, overcast lighting.
In the last 40 or 50 weddings I've done I can only remember one time when the guests were told not to use flash, so even if I'm not allowed to use flash, usually cell phones are popping all over the place.