It's a mix of both. If you sit in the same obvious spot every single day, then it's customary for others to let you sit there. Some people who hastily come in won't remember and might take your spot on an occasion or two. Just try to get there early to get your spot, but the bigger the class is, the less likely your seat is guaranteed. In busy lecture halls I would have to switch up my spots every single time, but I tried to remain in the same seating area whenever possible.