for the life of me, i cannot imagine how styro would cause a tank to crack - it would of cracked without it also....
styrofoam DOES NOT cause pressure points - its the other way around - thats the reason behind using it in the first place.
as far as actual usage goes - it depends on the size of the tank - anything over 100g - i would use Styrofoam, also if its a taller tank, i would use it even if less than a 100, but there is no good reason not to use it - if your wood is not even, it would create pressure points for the bottom of the tank.
if you have trim around the bottom, that means that your tank is not really big enough and the manufacturer knows that its safe for it to rest just on the trim, which kinda limits the Styrofoam funcationality- but event hen, if you do get a softer styro, it will help you level the tank.