when empty some of my 40B tanks kind of wobbled like they were resting moreso on two opposing corners. probably off by the thickness of a sheet or two of paper as well. But with water in they straighted out and sit flat. Youd be suprised how much glass flexes.
So if it sits flat when completely full then It will be fine. if not then I would shim it. foam is good like the person above said because it will fit the space easier.
Just for fun, try putting a square or a 4' level against the front piece of glass when it full!