Styrofoam doesn't level things: it helps compensate for unevenness. If the two ends are level and there is a slight warp between them, some grit, any uneveness, then foam will make a more even bed.
If one end is a half an inch higher the foam isn't going to compress more on the lighter (higher) end to even things up.
Being out of level looks funny when it shows under the trim and can stress the seams in the tank, increasing the chances of a leak. I'd level it much as possible with shims. Foam cut to shape might help. The seams are pretty solid and folks seem to get by with slightly out of square tanks: there is some tolerance but if your tank is not level you're putting yourself at an increased risk.