It really doesn't matter if you wear a tie or not, I think whatever you feel more comfortable wearing is what you should wear. I went to tons of events like this last year and I would say that 80% of people were business casual -- khakis/slacks, button down shirt or polo shirt. The other 20% were either horribly under-dressed or people who were over-dressed because they were coming from work.
I personally chose not to wear a tie because during a lot of these events you have a chance to talk with students, and when they're sitting there in jeans and a t-shirt, you feel very much like you're "on the outside looking in" because you're more dressed up than them and it highlights the difference even more. So just khakis and a button down shirt or whatever will make you look well put together, but not like you're trying too hard to impress.