I dislike tip jars, anywhere. They are tacky in that essentially they are asking for tips. Tips should be given, never requested.
That said I *always* tip my waitress and my bartender at a wedding, discreetly and just once. 15 years in the event industry for 3 different companies has taught me a tip stays in the hand its put in. Regardless of verbal instruction or labelled envelope, whomever the host hands it to, keeps it. I trust my friends to *try* to tip staff as hosts, I just doubt ther success.
At events other then weddings, like pay at the door, sponsored open bar inside, or a corporate event, community organization fundraiser or a private party I tip/don't tip on a case by case basis.