I usually bought them after, but that was because it was pre/early-internet and we didn't get the syllabus before the first day of class, lol.
I don't expect my students to have them on the first day of class though. If I really wanted them to read something, I'd email the class and let them know. But there are always changes (drop-outs, new registrations) the first week, so I just write it off basically.
I usually went ahead and bought them before, but would listen to the professors on the Fitzgerald class and return anything I could tell we wouldn't need. This wasn't pre- internet, but in 1999 I wasn't as internet savvy and a lot of options that I would have totally taken advantage of today weren't available then.