It would depend on how many you're applying to, how far away they are, and how much $$ you have! I think it's a good idea to check out a couple med schools and med centers if you've never been to them (if your undergrad doesn't have one) so you have a basis to critique what you see later on and figure out if it's right for you. I certainly wouldn't advise flying all over the country to your top 30 schools (that's how many I applied to!) because a) no time, b) no money and c) you get to see the schools if and when you interview.