My recommendation is to use the cove material, having a 90 deg. corner is not the best of choices, the cove transitions the load (Weight of water) better and also gives you piece of mind that all that weight doesn't push through a small crack left between foam and wall, if there is even a small gap, the weight of the water will eventually push the liner in there as if it was saran wrap (with or without duct tape), so the cove would cover any such potential spots.
Also, a cove gives a better transition to allow your kreepy crawley to climb the walls.
Not necessarily what I was hoping to hear (LOL), but good advice all the same. My husband seems to think he can construct a cove out of our leftover styrofoam. He has some kind of tool that will cut angles out that way. Maybe we'll go that route. I am so OUT OF MONEY it isn't even funny, so maybe we'll attempt to do the cove that way. I have to go out and buy a ladder today because the one that came with our pool is a deck ladder (which is perfect except we're not putting the deck on until NEXT year). I called the local pool store and they said they are $175!!! YOWSA!