How much is left below the "minimum insertion" mark? That will be the important part. If there is a generous amount, then I don't see 5mm making much difference in normal circumstances.

However, having said that, it's not so much the seat post itself, but the frame material, and whether the seat tube is capable of sustaining the leverage that will become more localised than with a large amount inserted.

If it's steel, probably OK. But if it's aluminium or carbon, there may be a risk. With carbon, if there is a reinforcing sleeve of aluminium bonded into the top of the seat tube, you may also be OK.

Then there is the expansion slot. How far does this extend down the seat tube? If a long way, and your seat post doesn't have much meat below the mark, then you might have an issue.

All said and done, 5mm isn't much for the seat post itself, but you will have to assess the effect on the frame.