Only in these two cases when formatting is never needed:
1) If you update to a more recent/latest Stable ROM by Xiaomi.eu, from a previous Stable ROM by Xiaomi.eu = No wipe or format needed
2) If you update to a more recent/latest Weekly ROM by Xiaomi.eu, from a previous Weekly ROM by Xiaomi.eu = No wipe or format needed
In the other cases, it's recommended (or mandatory) to format data.