If you lock your seed you are worse than unit tests - odds are you are not testing some interesting cases ever. The whole point of PBT is there are some properties that hold in all cases (well technically in the domain of the function inputs), so try random examples to see if I missed something. Generally the domain of all possible function inputs is a very large set such that exhaustive testing is impossible. The more different random seeds we try the better the odds we eventually catch some case that we didn't handle correctly.