I had never flattened prints that way before, it works very well. I suspect that for 'good' prints one would want to protect the print from both the platen and the foam with some textureless material. Mat board?? I guess one also needs a "cold press" like a piece of metal or something to hold the print flat while it cools.
That's exactly what I do - a piece of archival mat board on the top and bottom, and then the prints go into a large coffee table book of Ansel Adams. I hope maybe some good karma rubs off onto my images as they cool.