I am about to implement Portfolio for Jira on an existing Jira Cloud instance, with many projects and loads of existing data.
The guide does not mention any safety steps to do before installation (such as backup).
What is your experience? Do I need to bother with backing up Jira data, setting up a maintenance time window for adding Portfolio, etc.?
Or is it a simple plug it in and go sort of thing?
No need to backup Jira before installing Portfolio. The installation generally runs smoothly and won't affect your existing Jira installation until you start using it.
However, before you make any major changes using Portfolio, it might be nice to create a backup then. Once you commit the changes it can alter many issues at once, and if you do it by accident you'll need a way to roll back.