Here is the government website that says so and its translation (with a really long url, but the translation is better than that of Google Translate).
Appearantly every shop should have stickers telling customers that they round cash payments. I've never noticed that anywhere.
https://www.rijksoverheid.nl/onderwerpen/geldzaken/vraag-en-...
https://www.deepl.com/translator#nl/en/%20Mogen%20winkeliers....