the 90% principle, or do I have to get rid of everything I don’t use?
You of course have physical constraints in your (new, or current) house which will shape what to keep and what to purge. Let’s set those aside for now.
To answer the question at hand: Of course not. Expecting 100% adherence to a useful principle has a bit of a whiff of totalitarianism, if you ask me. The 90% principle states that a good faith effort that gets you 90% of the way to a worthy goal is a damn fine effort, and a damn fine result. At such a time congratulations - not recriminations - are in order.
An organizer’s business should have an organizing principle, right? Mine is pretty simple: to give my clients exquisite value for their money and time. And to have fun working together doing it.