NEED a wad, no, not even with a projectile, but do you want one??
I load loose powder, instead of prepared charges...
My .72 cal just happens to be bored to 14 gauge shotgun dimensions so I have the option of vegetable fiber wads. I sometimes use a wad to make the cannon sound louder. Sometimes I just pack the powder charge to make the cannon have a deeper sounding boom.
As for wadding with projectiles, if I am shooting a single projectile, no wadding is desired (pressure and accuracy concerns) if I am loading a shot charge, then I use aluminium foil, and an over shot card like used for muzzle loading shotguns.
Hope this helps