IMO no matter how you carry always place the blade against a pocket seam to help minimize this type of accident. I carry both ways based on whatever is lower profile, either RFP(tip up) or RRP(tip down), with no problems no matter what I am doing.
I think most people who hear 'tip down' carry dont take this into account. Always put the blade against the seam, and you wont have any problems. Personally, when i'm doing tip down, its either in my back pocket, or on the upper portion of my front pockets, blade against the seam both ways.