I went through this just recently.
My thought process was that, if I could get all the tanks in some kind of rack, I could carry more because they'd be standing.
This is true to some degree, but there's a couple of problems with the idea:
1. Most any sort of "really" secure rack is going to be some kind of permanent installation, meaning that on days where I want to carry something other than tanks, the rack will be in the way.
2. Standing the tanks upright might seem like a good idea, but they're much more inherently stable if they're lying down.
3. Having a rack that stands tanks upright means that I'm limited in the number of tanks I can carry - if I have 10 "racks," for example, then I can only carry 10 tanks before resorting back to the "lay them down and secure them" method that I used before I got a tank rack.
4. Having the tops of the tanks exposed higher than the sides of the truck attracts unwanted attention - they're simply more "out of sight, out of mind" if they're lying down.
5. Unless a (vertially-oriented) tank rack is cut to exacting sizes of the tank, there's going to be a lot of wobble in the tanks, possibly doing damage and at least making the whole driving thing a noisy excursion.
6. Cost.
7. Permanent alteration to my vehicle.
I tried a variety of methods of securing tanks, but finally found my favorite way of securing single tanks:
http://www.scubatoys.com/store/detail.asp?PRODUCT_ID=FoamTankRack Cheap, easy, simple, and they come out of the truck when I'm done. Best of all, they prevent both roll and back and forth sliding, as they're sorta "grippy."
They also fit most scuba tanks without alteration.
Simple.