I love suits and ties and the whole shebang, but if it's not such a formal party, I don't think a tie is necessary. In my experience, most guys will avoid ties like the plague. True-life overheard words: "Do I really have to wear a tie at my own wedding?"
I suggest doing a search for Neofinitia's posts on the "What are you wearing" thread; he oftens makes a suit and tie look casual through offbeat color coordination and accessories. My favorite touch is a frog brooch he once wore on the neck of his tie.