Why not just behead the bounties?

Good question

History isn't helpful since actual bounty hunters were rare and usually got their kills confirmed at the time they happened

Guess the only answer is, it's the way TV and movie westerns did it, and Tarantino's characters are bound by those laws, their creator having absorbed all of cinema history in one continuous download

EDIT: I read a historian's account regarding old west law enforcement. A lot of bounties had specific measurements and body markings recorded when they were first arrested and only line drawings of faces.

Heads would likely be the least valuable form of identification, in that case.