When an audit is worth your time.
Four triggers justify a full backlink audit. A manual action for unnatural links makes it mandatory. Buying a domain or site makes it due diligence: you are acquiring the link profile, including whatever a previous owner bought. Taking over from a vendor whose methods you cannot verify is the third, especially if their reporting never let you check individual links. The fourth is a traffic decline that coincides with a confirmed link spam update.
What does not justify one: routine anxiety about "toxic links." Toxicity scores are tool inventions, and Google's systems already ignore most spam pointed at most sites.