there are only a hand full of reasons to mount one there..

they are normally smaller than stock capacity.. so you are loosing ability to carry as much gas, so they are comon in drag race cars because it takes the tank off the bottom of the car allowing for more straight through exhaust, or suspension mods...

sometimes they are added on street cars, for indipendent fuel systems.. for things like nitrous... ex: seperate tank with higher octane race gas that is ONLY pumped under nitrous use..

or they use a switching system to be able to adjust boost on turbo cars and run race gas..

so no.. as i can see it.. you don't need it