I'm assuming you're getting some that came from a surplus store or from auction? I would be more worried about the contamination present in most of those. The previous fuel content won't affect the gas put in, but most of those 20L cans spend most of their lives either strapped to the back / sides of vehicles that are exposed to severe environmental conditions, or sit in racks outside year round. It's not unheard of to pick up a gerry can of fuel in the winter, and it be frozen solid from the water ingression (there's a reason we use diesel conditioner lol). It's also not unheard of to be filling a piece of equipment's fuel tank and large junks of dirt / mud come out of the can (fuel strainers are a great thing).
Anyways - the point of this is that I'd be making sure they are clean before (re)filling them. I'd also be looking to make sure there's no hidden damage. You'd be surprised how easy it is to miss a small puncture in one of those - like I said they're exposed to some pretty brutal conditions. Tag straps really don't mean a whole lot to anyone, unless they deal with that type of thing daily. A couple jugs strapped to your rig really doesn't meet the need like having a 10T full to the brim with a mix of fuel cans where the tags determine whether or not some poor soul breaks down in the middle of nowhere from the wrong fuel put into his vehicle or not :stir: