hi, eda_ak & jjww110,
In fact, the wire load model or the back-annotated information are two choice in STA. In pre-layout, you don't have the back-annotated information, therefore, you have to use the wireload model ( often very inacurate ). When you have your layout you could export the back-annotated & parasitic information to your STA. That's to say, in fact, PT also use it in the post-layout STA.
About the re-syn, if you got a very "bad" library, often all the wire load is set to be simplily 0, you should follow eda_ak's words, updating the wire load model in library by what you got in the layout and re-syn. But in most cases, the wire load model provided in the worst case library are very conservative, far worse than you get in layout, in such a case, you gain little by re-syn.