A few people mount the good 'ol metal 5 gallon fuel tanks horizontally (e.g. on roof racks). Doing this still makes the fuel level hit the cap seal. When one does this, is only 3 or so gallons placed in these tanks or are they fully filled?
If you're using the NATO style cans you should be good for the whole 5 gallons. These things seal up tight. Sometimes that are quite hard to open do to the presure difference inside the can vs outside the can. Now if you are talking about the Blitz cans, I've heard that those things never seal quite right. In this case you probably don't want them full.
Use cans that seal and it won't be a problem. Nato cans (and their clones) and the new US military (plastic) cans sel up great. You can stow these upside down even.
Roof rack is a terrible place for heavy stuff like gas cans. Put the light stuff up top. Put the cans on a rack/mount on the bumper. Personally I'd put the fuel cans inside the ig before I'd put them up top.
I bought one of the carriers that Slee sells, bolted it to my roofrack and tried putting the s***z cans up there... what a mess, especially the "CARB approved" ones, gas dripping everywhere! (had one of each, the "CARB" one totally sucked, and it's great fun trying to fish the stupid spigot out of the can when you don't set it up exactly right...!) Got ahold of a couple of NATO cans, filled 'em up, slapped them up there, and not a drop leaked, even in 100+ heat, for weeks. Plus, the spouts actually dispense the gas at a reasonable rate, and don't leak on your shoes, and, well, I feel a rant coming on... need