do separate databases have any impact each other?

While writing installation instructions for my new PostgreSQL product, I found myself
writing the following sentence:
"For first time users, we recommend building the gnova database,
since it has no impact on other databases."

Is this really true? Of course, my gnova database will take some disk space,
but other than that can y'all think of other concerns a big production database
admin might have about installing a new (smallish) database?
Is it true that if no one has begun a session involving my database, it simply
sits on disk and cannot possibly interfere with other databases/sessions?
When a session ends that had opened my database, do all traces of it disappear,
except its life on disk? How about .so's it might have caused to be loaded?

TJ