My seat only goes about 2" lower and then the end of the seatpost does into the shock, so no I don't lower my seat when I do go down sketchy dh. If I'm gonna eat it, well then so be it, nothing I could do to stop it.
If I'm specifically going out to bomb hills and do drops then I switch out my seatpost/seat for a shorter post and the big 'ol padded seat that came with the bike.