We never like to see a “tip cup” on the bar at a hosted event like a wedding, but this might be an exception that warms everyone’s hearts and does some good in the world.
Advise your bartenders that soliciting tips is not what you had in mind,and take care of them ahead of time. Then station a big brandy snifter (or dog bowl!) at the bar, and a photo or two from either this wonderful charity that does beagle rescue (http://www.beaglepaws.com/news.html), Unicef(www.unicef.org), or any other cause that is dear to you. At the end of the celebration, you’ll have a great donation,and you can thank your guests for their generosity in your thank you notes.