Apparently all the Ghouls which are classified as SSS have a kakuja obtained by cannibalization. Does a ghoul need to cannibalize to reach SSS level? is this mentioned in the manga, some OVA or something?
The only 3 ghouls considered as SSS are kakujas (see article for reference). Also, in most cases, power of ghoul is determined by his kagune properties (which can become weapon, full body protective armor, etc), so kakujas clearly have advantage in power over regular ghouls. So, as we don't have any other facts regarding rating assignment, I'd say - yes.