You would sign the FSS last, as signing it means you have checked/confirmed that all monies are correct and available.
For example, if in the source funds section it states the 'available' funds at the top you should also check the tab to ensure the funds are actually ready. I had a settlement take place yesterday where the source funds was showing the amount, but when I clicked the tab it stated the the funds were not ready. So I had to wait for the other side to verify and authorise the funds. Once that was done I was able to confirm that all funds were correct, ready, and balancing, and therefore could sign the FSS as correct.