Should I cut control joints into a 10' by 12' shed slab?
This is largely a matter of opinion, but in my experience a 10x12 slab will almost certainly crack. There's no question but that I'd cross-cut it, at least. I might even go so far as to split the length into thirds. Clean, straight lines are preferable to jagged ones, and you can always fill your cuts with caulk to keep the floor surface intact.

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