I visited my program before I applied. I wouldn't say it's absolutely necessary but I think it helps (there was a question of the application that asked if you visited, but I doubt that is a significant factor in admission). If you were applying to a place that you were familiar with it might not be a huge deal, but if you are planning on applying to a place on the other side of the country it might be a good idea to check it out. For example, if you grew up in San Diego, but are applying to a school in New England you might want to check it out during the winter (I used to think this was funny until my Californian wife bought every piece of winter gear she could find after she saw a snowflake when we first mover to Colorado). Or if you grew up in a big city you might end up hating going to a school in a rural area. I think you get the picture. PT school is usually about 3 years long, which is a long time to be stuck somewhere you hate. As far the actual tour itself I'm not sure they are always super useful. During those tours they always put their best foot forward and show you all of the new, shiny stuff and introduce you to happy, chirpy students. I just don't think it's generally an accurate portrayal of the school. Have you ever seen a picture of food from a restaurant that looked amazing, but once you actually ordered it you end up with a plate of garbage? Kind of like that.
I wouldn't say it's the most important thing. Certainly, if you are applying to schools in your immediate area, go visit. But if it requires a plane ticket or drive, just wait to see if you get an interview. You will get the grand tours on interview day. If the school doesn't do interviews, then plan a visit if/when you get accepted and are trying to decide between schools.