I think I've read the recommended place to put BindableEvents somewhere but I wasn't able to find it again.
Thanks to Formidable_Beast and a little further research, I've found out that ServerStorage can only be accessed by the server, and that ReplicatedStorage can be found on both client and server. That means, it makes sense to place a server-server BindableEvent in ServerStorage, while a client-client BindableEvent in ReplicatedStorage.