I think that butane extraction would work best with the plant matter dry. That way the liquid butane can soak into the dried plant matter, solvate the THC, and wash out with no interaction with or competition from water. Sometimes "solvent pairs" work better than one solvent alone and I suppose at some level that might theoretically apply to wet flowers (i.e. that some moisture might be beneficial somehow).
The only time I did butane extractions the flowers were at the usual, recommended 62% RH, which seemed to work fine. I would guess that 50% RH would be better, though...