If done correctly.....most bands are hired months in advance knowing the band's repertoire and any song requests are done then so the band can prepare. Agreed, no songs on the fly unless the band knows it. And be prepared to have folks sit in. It's all part of the game. Best part is the money.We've only played a couple weddings, but made it clear to the groom/bride that we are a cover band, here is our set list, and generally would not be able to take requests on the fly (we are not pros). The ones that hired us knew our set list and had heard us play before. Since we're not a typical wedding band, we charge a lot less, too. The couple weddings we've done went very well, would definitely recommend that experience if it comes your way.
I forgot to add.....ixnay on the jibber jabber! Music, music, music!! Medleys are always good.