I would never carry an assisted flipper folder tip-down, that's just an accident waiting to happen. I think people sometimes get confused since more and more knives these days are drilled out for all four carry positions. The only safe way I can imagine to carry an assisted flipper tip-down, would be if you carried it in your back pocket, or if you reversed the clip orientation e.g. clipped for LFP tip-down but carried in your RFP. Even without a flipper, accidents can happen carrying assisted knives tip down unless they have a safety mechanism like the one SOG uses. No matter whether you carry up or down, the spine of that blade should always be sandwiched against your pocket seam to avoid accidents. With unassisted knives, there's not as much need to be picky, it's all up to personal preference. Even manual flippers, like the CRKT M16 & M21 seem to be okay for tip down carry because of the ball detent. The problem with assisted knives is that they usually are held closed only by pressure from a torsion bar, which can pop the blade open with very little pressure. I can only speak to my own experience, but I bump into door frames & other obstacles with my hips often enough that this sort of accident would really worry me.