Interesting thread.
I get REAL nervous anytime I'm carrying propane with me. I have two 100# tanks and three 30# tanks.
I fill them all at the beginning of fall and keep the big ones in reserve in case we get snowed in. I toss the little ones in my trunk. They've never said anything and it's maybe 6-8 miles from there to here. I come straight home.
Sometimes I'll also have 30 gallons of diesel with me (5-gal containers). As I'm driving down the road, usually alone, I just dare someone to backend me because it would be the end of both of us!
When I carry tanks, this is what I do to keep them from rolling. I have some treated 6x6's that are maybe 18" long. I set the down (yes, laying down

) and put the 6x6's on the outside of it. I then take some of those small ratchet straps and winch the group together. The 6x6's on the outside keeps things from rolling around and the straps keep everything together.
This works with a single tank very nicely and I've never had a tank rolling around. When I get two tanks in there, I ratchet them togethter as well, even the handles of them. Locking the handles together (for me) has kept them from rolling around.
I'll use the truck if I can get it. I don't like the idea of propane in my trunk nor the idea of it so close to me. I also realize if the tank was in the bed of the truck and somehow exploded...it wouldn't really matter if I was in the truck or my car. I figure I'm gone either way.