Supplier qualification for a small purchasing team

Full supplier qualification — questionnaire, document review, site audit, trial order, periodic re-assessment — assumes a procurement function with people to spare. Most of our clients have one or two buyers doing this alongside everything else. The usual compromise is tiering: full assessment for suppliers whose failure would stop production or harm someone, a document check for the rest, and a re-assessment trigger based on performance rather than a calendar. Where do you draw the line, and has a low-tier supplier ever caused you a high-tier problem?