I don't believe the flattening would cause significant changes to the Amsteel's strength. Amsteel is commonly used for heavy (12,000lb+) recovery winches, and when using the synthetic line a Hawse fairlead is used. The Hawse is essentially just a smooth piece of metal the Amsteel slides over, and when it does so the Amsteel is flattened. This flattening has not, to my knowledge, been shown to reduce the strength of the Amsteel in these situations, even when loaded nearly to breaking strength.
I would consider abrasion from rough surfaces to be far more worrisome than flattening, especially considering the weight-rating safety factors typically designed into our hammock suspensions.