You delete a post when the content no longer adds anything to the site. Posts are closed for a variety of reasons, so let's look at each close reason and whether they should likely be deleted:
##Exact Duplicate:
It depends; Look at the context of how they are asked.
You'll want to keep the post if the wording provides another way for a search query to find the content. It's a bit like a "see also …" entry in an index.
Delete it if the duplicate does not add terminology or alternate phrasing to find the question. It may not be worth cluttering up the system with this exact duplicate. There are several poorly phrased questions that will not direct traffic to us.
##Off Topic:
Almost always delete it.
Off topic usually says "This shouldn't have been posted here in the first place." So, not about Ubuntu (read, Mint, kali, Debian, Fedora, etc.), solved bugs or bug reports, no reproducible stuff that nobody knows why or how was the issue solved, or EOL, just let it burn!
##Too broad:
Delete it if it says "this is nothing but a 'broken window' and we should get rid of it." This includes list of question where the only information is about 2 lines of text per answer or wikipedia/internet dumps.
##Unclear: Always delete it.
Closing it before deletion provided ample time for the author (or community) to fix it up. If it wasn't re-opened by this time, it should probably go.
##Primary opinion based: See "Too broad".
This was adapted of Robert Cartaino's answer since some old terminology (not constructive, NARQ, Too localized) is no longer applicable. Also note, that an answer that is not an answer (if you strip the markup, doesn't answer the question) is not a party stopper to delete it.
