Well, I got to chat with an Apple rep on the website a few minutes ago, figured I'd post this so you all know for the future.
I pretended to be a concerned parent for this scenario, just in case there's any confusion.
Tim: Hi, my name is Tim. Welcome to Apple! How are you?
Me: I'm good, I have a question about purchasing AppleCare for my son's MacBook
Tim: I'm happy to assist with that.
Me: Okay, well, he's had the MacBook for about 8 months, and we bought it from an Apple Authorized Reseller (Micro Center, specifically), and he has a band's sticker on the top of the case. Can he still get the AppleCare, and does the sticker void the warranty?
Tim: He can still get the AppleCare. Is he a student?
Me: Yes, he's currently a University student. So, the sticker hasn't voided his warranty?
Tim: A lot of students do that. As long as it doesn't cause the issue and isn't permanent, it's not an issue. If he were to carve the name into the case with a pen knife, that wouldn't be wise.
Me: Haha, I can't imagine it would be!
So, for the definitive answer, no, the warranty is NOT voided by a sticker, as long as it doesn't interfere with normal operation of the machine.
Woo hoo!