inertia tensor

i am looking for a little help or maybe a more clear explanation about the inertia tensor. i am doing a simulation with some cubes and i fully understand how the moment of inertia about each axis is calculated and such. however i am confused with the tensor. what i think i understand is that i do not necessarily need the tensor represented as a 3 by 3 matrix to correctly represent the moment of inertia if my rigid body is completley symmetrical about each axis of rotation. like my cube, or a sphere or a rectangle. if the body is completely symmetrical about each axis in local or object space than i can represent its moment of inertia in my simulation as simply a vector, or in this case as a simple scalar actually since for the cube this value would be the same for each component (x,y,z). is this correct then??