I don't think full time teachers realize how much we subs actually do need seating charts. I understand why they might not. They know the kids. But if we're walking in completely fresh, how are we supposed to know who is supposed to sit where?
Yesterday I had a chance to talk with the teacher before the day. She was going to be on campus all day. She apologized for not having a seating chart, and I was obliged to wing it.
She walked into class at some point during the day, and sure enough, a couple students were not in their proper seats. (This got them into trouble, not me. Hey, they were warned that she might stop by.) And when two boys attempted to later switch seats (blatantly in front of me), I made sure that the teacher knew of it. (I wouldn't want to be either of them today.)
Kids always try to get away with something with a substitute teacher. We need all the tools that we can get to combat this.