Do you show a CSV import preview before writing customer data?

A small SaaS import question about preview screens, rejected rows, duplicate handling, and support cleanup cost.

For CSV import in a small SaaS, I think the preview screen should appear before the product writes anything into the account. The useful version does not need to be a full editor. It can show row counts, matched columns, duplicate warnings, rejected rows, and a downloadable error report. The main question is where the support cost starts. If a failed import requires manual cleanup or an engineering ticket, the preview is probably cheaper than fixing bad data after the fact. How much detail do you show before the final import button?