Visiting the campuses of the colleges and universities where you are interested in studying is not required for admission. Visiting at least a few colleges, however, will really help you get a sense of what you can expect and where you'll be happy. It also signals to a college that you're interested, and admissions officers are looking for applicants who are excited about their schools.
Summer vacation is a convenient time to visit colleges, but bear in mind that visiting an empty campus will make a very different impression than visiting a campus filled with students. You probably won't be able to visit every school on your list while classes are in session, so here are my three tips: