There is no one “right” answer.
What you do not want to do is run out of disk. On a system with a large amount of storage 98% used might still be saying there are GBs/TBs of storage available. On a system with a small amount of storage 98% used might be saying system termination is imminent if a run away process starts consuming storage.
IBM ships the systems with a default value of 95% (though it’s done by setting the system value QSTGLOWLMT to 5% available for use).