Cast Iron Insert Fireplace.

I'm about to install a CI fireplace.
I did one about 10 years ago and filled the gap behind with leanmix with a strong cement mortar at the top to form the sloping outlet from the actual fire. This was necessary so that any unburnt debris slide back into the fire place and didn't accumulate behind.
Now the new one has an access point at the base for hoiking out such accumulations.
Does this mean I don't need to infill behind?
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