I think campus visits matter at schools that stress "fit" So schools like Tuck, Fuqua, Yale SOM, Darden, etc. and less so at other schools. However I spoke with my buddy about NYU when i considered it and he said they care a lot about what you did to get to know the school like visit, and what not. I think if you can visit definitely get out there to all the schools. If not, make sure you visit the information sessions that are in your area. This may be a bit late for this cycle, but if there are some sessions in Jan def do your best to make it out. They remember everyone who shows up.
In terms on when in the process to visit, if you are going for an informational type visit and not an open interview I would go after you have done a decent amount of the application to really have questions that you need answered. If you are going for an open interview, i would have your application as close to complete as possible so you know what to talk about during your interview.
Hope this helps.