The European Medicines Agency (EMA) maintains a site with answers to frequently asked questions regarding GMP. This site has been updated this year and now also provides an answer to the question with regard to the mandatory use of metal detectors in tableting.
Metal particles may come from raw materials or, as in the case of tableting, from the equipment - for example, due to damage of the moving metal parts in a press. In cases where this is possible, the Agency recommends the use of metal detectors. If there is no routine use of metal detectors, the company needs to demonstrate that their use is not necessary. This might be difficult in practice.
Find this and more interesting GMP answers on the EMA's Q&A site.