backfill on cast iron insert

Hi

Just had a cast iron insert fitted by a local fireplace company.

Asked them about backfilling behind the cast iron (the previous one that is being replaced had been backfilled) and was told there is no need due to a gas fire fitted into the insert.

remember at my old house when we had a cast iron insert fitted, which had a gas fire that was backfilled in the builders opening.

So is there any need to be backfilled (vermiculite and cement)?

thanks