The privileges description for 10k rep has a pretty in-depth explanation of how deleting posts works, and it elaborates on specifically when you should consider voting to delete a question under the subsection titled "When should I delete questions?". I find this very helpful, not only for my own guidance but also because I can point other people to it if necessary.
Now that the new
/review pages contain both "Delete" and "Recommend Deletion" buttons for low quality posts (determined by your rep as I understand), I think it would be beneficial to have a similar sub-section in the 20k privileges paged entitled "When should I delete answers?" The current version points out that you gain this power upon hitting 20k rep, but it completely glosses over it and provides no direction on how to effectively (and responsibly) use this power.
Although presumably we're trusting the "trusted users" to make good judgments here, I think that at least having some kind of basic guidelines codified would be very beneficial.