If there are voids on the OUTSIDE - then yes, fill those in w/ high-temp mortar.
If there are voids on the INSIDE - and they are deeper than 1/2", then go ahead and fill those in w/ high-temp mortar.
If they are LESS than 1/2" in depth, and the oven has NOT BEEN FIRED YET, go ahead and fill it in - but keep an eye on that area.
If if comes out, scrub or scrape that area clean and apply Rutland’s high-temp mortar for chipped mortar -