There's two ways to think about this problem: routes advertised and routes received.

Received: So for the most part people implement zero or just minimal filters on routes received from peers. They might drop a set of ASNs they consider large that would be indicative of a leak. Some may go the extra distance and even do IRR filtering. But for the most part people are fairly permissive in what they accept from peers

Advertised: Here's the catch. You announce routes to HE and they're propagating it to networks you don't anticipate. This pulls in traffic from other HE peers you weren't expecting. You don't really have much controls here. You can try to prepend but remember it's the other leaked peers of HE that will generally set a better local pref to HE (by virtue of being a peer) so your prepends won't do anything.