Lead source is the field most teams care most about and govern least. Cleaning it up is one of the highest-ROI MOPs projects you can run — and it is rarely glamorous.
Original vs latest
Maintain two source fields on every lead and contact: original (set once, never overwritten) and latest (updated on every meaningful touch). Reporting that uses only one of them will tell half the story.
UTM governance
Standardize lowercase, dashed values. Maintain a short list of approved sources and mediums. Reject anything else at the form layer. The day you let one campaign use 'Google_CPC' next to 'google-cpc' is the day your dashboards become opinions.
The field architecture
On the lead record, capture: original source, original medium, original campaign, latest equivalents, first form, first touch URL, and first touch timestamp. Push these to opportunity at conversion. Now sourced and influenced pipeline are real numbers.