Ask HN: Customer endorsements / use of customer logos on website

This post [1] created an interesting discussion that seemed like it should have its own Ask HN. It seems that quite a few young sites will post the logos of its customers (potentially implying endorsement / commercial relationship). Are there best practices for doing this (if at all) and what advice would the HN community have for a startup seeking to promote itself by use of customer logos.

1. https://news.ycombinator.com/item?id=21998632