I'm on a waitlist and I visited the school.
I think it's a good idea--if you are accepted off of the waitlist, it's good to know whether or not you like the campus. If you don't, then you may not want to go there.
It also shows that you are interested in the school (you should follow it up with a letter of continued interest).
Of course, the down side--if you really love the campus, you may have problems imagining yourself at the law school that you will otherwise be attending. It's a risk.
Lots of luck!