The mirror seems to land pretty gently on a padded surface. There's a kick when the shutter closes, but that's post-exposure.
The slap and shutter vibration are not insignificant, however. My Press Graflex (non-rotating back) has a tripod socket on the side for verticals--rather impractical to begin with using the focusing hood, but manageable with the groundglass back--and even with my biggest tripod and head, you can usually detect some camera vibration in the negs. Maybe sandbags would help. For horizontals, though, it isn't an issue.