I often book across seasons and have had no trouble at all doing it online. The only time you may need MS to help book is if your stay crosses use years.
For example, if I wanted to stay 11/27 - 12/4 and had a December use year, I might have to call MS to add the December nights after I booked the November nights online when the 11 month window opened. To book the December nights online, the system (due to a glitch) would make me wait until the window opens for 12/1.