For Weddings there really are no rules. Each wedding is unique, insofar as the Bride & Family decide on the structure of the event. There are, however, some typical places in the ceremony where piping is often performed. You may choose any or all of the options:
1. Playing as the wedding party enters. Usually this would be a stately processional song played until the wedding party is in place at the front of the church.
2. As the Bride comes up the aisle.
3. During Communion.
4. During the lighting of the Unity Candle.
5. As the Bride & Groom go down the aisle at the conclusion of the ceremony.
6. Outside the church (weather permitting) as the families & guests exit.
Occasionally, piping is requested prior to the service as the families and guests arrive. This is certainly fine, particularly when you do not plan to have piping during the service (you don't want to overdo it!). If you plan to have the Bride piped in, it's probably best not to have any piping prior to that in order to maximize the impact of hearing the pipes for the first time.