In Part Three we’re required to make our own stories. We’re given the main primitive meaning but many primitives have more than one meaning.
If we don’t look up the primitives in the index and check for alternative meanings we may miss an opportunity for a richer or simpler story, both of which could help with remembering the kanji.
Last time I went through Heisig I didn’t use the index once; I think that and an over-reliance on the RvTK site made learning harder.
The aim is to construct images/stories that are vivid to you. To do that, first allow yourself all the meanings of each primitive. Next, don’t look at other’s stories unless you’re stuck; once you read them your imagination may be biased towards their interpretation and you need to form your own.