Typically, they don't.
I know and respect a lot of guitar guys, better than me, who think a shim in the front, or one in the back, creates some magic geometry. Their guitars play and sound great but I don't shim. I've got scores and scores of bolt neck guitars and presently none have shims. I view shims as triage; a way to get a guitar back within the range where it can be adjusted. I'm more likely to switch necks A and D with bodies A and D, instead of "forcing" as it were a neck to stay where it does not belong. Or clean the crap off the bottom of the heel or out of the neck pocket.

Obviously a '64 Mustang should stay united with its neck. (Ya'll should be glad I don't own one, I'd have taken off and lost that little neck decades ago.)