The larger the tank, the more important it is to be perfect level. This is especially important in reef tanks, because we have so much water flow. If the tank is uneven, it will add additional stress weight to one of the edges/seam. This is compounded by the fact that we put wave makers and such in the tanks, and with the back and forth flow of the water, this adds to the pressure on the uneven side. Its only a matter of time before the seam fails and your tank ends up o the floor, especially in tanks over 100g in size
That being said. I never leveled my 75g, and it does favor one side a slight bit, but never been enough to be concerned. On the build for my 640g though, I was extremely meticulous in making sure it was perfect level in all directions. I even rebuilt the stand multiple times just to get it right because once its filled, it will be 6000 pounds of water being moved around inside it, and the last thing I want to see is all that water on my basement floor