I think it's preferable to go with the earliest tour of the day on a day with regular park opening hours (9:00) if at all possible. We went on the last Saturday in January on an 8:30 tour...the second one of the day. After booking it, park hours changed to 8:00. It was a complete madhouse when we arrived "early" in the park for our tour at about 7:45. The tour was fine once we got off of Main Street but at least the first hour we were dodging people and the guide was working hard to try keep everyone together. Loved the tour but that part was not so enjoyable. Also I personally wouldn't have wanted this to be our first day at MK.
If you are able to schedule your tour on a date that has EMM (not Extra Magic Hours) then that is pretty much a guarantee the park will open at 9:00. EMM lets in a smaller number of guests who paid extra to be in Fantasyland from 7:45-9:00 so you'd be fairly assured of a relatively empty Main Street as you begin your tour.