Actually the "best" place to put filled jerrycans is inside just behind the front seat (if rear seat removed) or behind the rear seat, for reasons of COG (center of gravity) and load distribution. Admittedly this is scary as far as having gasoline inside the car but new quality DeutscheOptik cans shouldn't leak. Put bulky low density items on the roofrack, like camping gear, folding chairs etc, lash them down well and cover with tarp for dust protection. Don't put cans inside car unless you have the metal ones that have positive sealing with a gasket and locking mechanism, don't risk leakage with plastic cans that seal only with plastic threads.
If you put 5 full cans on the roof you will absolutely feel the difference as far as handling.
If you do decide to put the cans on the roof, fill them at the last possible moment and empty as quickly as possible.
Reference: "Expedition Guide" by Tom Sheppard