I have a theory about spacers. I haven't bothered to post it before, because I expect that as soon as I do I will see a flurry of protest that I don't know what I'm talking about, "my oven hasn't cracked, you're an idiot, etc."
However, here it is, in all its glory: I believe brick spacers increase the risk
of cracks. This comment is based on personal experience.
Think about it. A lot of people mix a little clay in their mortar. About 1/6 of the mix if you use a very common homebrew much discussed on the internet.
Clay shrinks. Mortar made with clay also shrinks a little. Two bricks wedged apart by a little piece of brick can't move towards each other as the mortar shrinks. So, there is a sort of internal stress created in the joint, increasing the risk of cracking.
I reckon if you use spacers, pull them out when the mortar has set but is still wet. Just sayin'.........