If you add more pessimistic margins and close the design in the pre-cts itself you may not see any violations after post-cts and post-route.
But if you add more pessimism at the early stage itself, you might blow up the power,area to meet the performance requirements.In order to meet design goals with out some of the goals, we add some extra margin which is reasonable at the pre-cts stage and close the design timing (skew+jitter+ocv margin+pre-cts modelling margin + any other margin for example sign off margin).
If you try to close the timing post-route, after all our CAD tools designed by human beings cannot achieve the global or local solution of placement soln.. and routing soln.. simultaneously.
I hope u understood my argument.
Cheers
Teja