Never came to a definite, real-life answer to this myself. For precaution measures, I always try to keep at least 10% of the SSD space free. Other than that, I'd say that you only need more space if you feel that performance has decreased.
Disk cleanup might remove a lot of unneeded junk, so you might try it first, preferably in admin mode (where you can also remove old Windows installations if they are not needed, as well as old updates).