The first advice I would give the photographer and the couple to be is to ask beforehand what the policy is. Each policy is very peculiar to the Father, or Priest in charge. There is no common rule across the whole institution. Each priest applies his/her own view on the matter. And each person has a personal experience and will let the photographer move and flash according to the space allocated or permitted, according to the couple's personality or according his/her ethics.
I therefore agreed before the service started and got closer to the table where the parents and couple were gathering as they were signing one after the other. The trick is that you can pretend to photograph the other people has the priest is focused on his duty with the witness. But in fact you can steal quickly one or two shot while he is busy working. After pressing the shutter release I turn my lens in the other direction to pretend that I paying attention to the other scene. That trick is actually quite easy to achieve especially if you are working with digital. With film it becomes a bit more tricky as you really need to have decent light in the church in order to photograph without flash otherwise the priest would be definitely aware of what is going on.
The second photograph is the formal one the priest asked for. But most of my customers don't really like it to be frank. But as long as you know both the score, there is no one to blame and you play the game.
The third photograph is the sort of setting which is perfect for some sort of compromise between reportage and formal style. The space you are in is flexible enough to try different angles and to have some distance. It is a classic shot, but a honest and direct one. A bit of flash with the real actual signing in process.
Then he kisses his bride. Not posed, they really wanted to kiss. Do not forget that most of the time a piece of music is being played while we are signing. If the music has stopped for quite a while and you are still taking pictures, you are in danger zone. And the priest will let you know. Learn to be as efficient as possible is those mini moments and macro spaces, and work in order. Formal and important ones first, more creative and subtle in between or at the end.
Flash is always recommended as you do the formal pictures of the couple signing. It is important to use it to get the sharpest image possible but also to show off. Meaning that the parents always expect from the official photographer to have some professional equipment to be displayed and the pro flash is a good tool to make that statement. But also take as many atmospheric ones without it especially if you are working with digital. Try not to forget the witnesses too.
To conclude I would say that most of this sort of picture is very precious to the parents such as the formal group ones. You cannot afford to miss it. They are not really creative but they have to be dead right, perfectly composed and lighten up, and with good expressions. And you HAVE TO DO IT VERY QUICKLY as you have to get ready for the walking out now. No rest for the pro! Maybe 5 minutes in 3 hours time...if you are lucky.

