Ribcages are still easier to spot than lone skulls IMO, so it's always worth checking around them. Stumbling on a skull is slightly less probable, but it will happen sooner or later.

By the way, if you inspect the refinery, you can see that the device always uses sap as fuel of sorts, but the output isn't similarly defined. It could be that in future we can refine something else with it than just jotun brains, basically same way as smelters are right now.