That's correct (I had been wondering which alliterations you were noticing) - thence and thither are both voiced in most American dialects (I think one of his subpages talks a bit about dialectical variation within America). For what it's worth, in various less-formal/more slangy dialects, you sometimes get voiced /th/ -> /z/ and unvoiced /th/ -> /d/. Example's include "Howzat?" ("how's that?") and "who dis?" ("Who's this?"), but these are universally heard as uneducated/informal and/or associated with various ethnic accents.