Flash is usually required at some stage of photographing a wedding. It may be used as a fill-in source to reduce contrast, bounced to provide relatively shadow-free illumination in unlit corners, or fired directly as the primary source of light. However, there are often some circumstances where its use is either forbidden or requires permission. For this reason, it is certainly good practice to talk to the priest or registrar prior to the wedding to seek their views on the matter. A typical response is that limited use of flash is acceptable during the service or ceremony, except when the vows are being made or the register is being signed. At these important stages, it is vital that the couple are focused completely upon what they are saying or doing. Indeed, photography of the signing of the register, with or without flash, is not normally permitted. Almost all such photographs are taken during staged re-runs of the register-signing process. The key message is to seek advice in advance and observe any restrictions.
Other thoughts regarding the use of flash include ensuring that spare batteries are available. The flash unit is likely to be fired on many occasions, so batteries may be discharged rapidly. Also, it can be worth taking a long flash lead so that the flashgun can be used off-camera, perhaps to one side so that the shadow of the subject does not appear on the background. However, when used in this way, it can be difficult to get illumination that looks natural.