I did all my visits pre-application, with the exception of my backups. I found campus visits INVALUABLE in getting the right information from students - getting a pulse for schools, etc. It was very very helpful for writing about the school. Its hard to sound convincingly passionate about something when you havent even visited.. Talking about your visit and the people you met in an essay sounds much much more powerful in my mind than just talking about what you read on the website.
I personally suggest visits - and TALK TALK TALK to students - if you can afford em.
There's another benefit - if you get waitlisted - at which point yield management comes into play double time - you can at least write a letter about how much you love the school and sound convincing - if you've never been, it wont be especially meaningful.