Many wedding receptions have the proverbial toasts (from the best man, perhaps, or father of the bride). But increasingly the open mic is coming back to haunt brides and grooms.
An open mic can be a fun aspect of a wedding reception, and it's definitely a low-cost item that can lead to some good entertainment, but beware of handing the mic over to anyone who has "a word" for the bride and groom. Some comments you may not want to hear, however cutsie or well-intended they may be.
Use your open mic to add to your wedding reception, not detract from it.