Please see note below.
At 20k, you should downvote and vote to delete. Flags for moderator attention should be reserved for posts that you cannot handle yourself or require immediate attention. We want the community to be involved in moderation. From A Theory of Moderation (emphasis original):
[W]hat do community [♦] moderators do? The short answer is, as little as possible!
This is for the same reason we want 3k users to vote to close rather than flag when possible. See How should my flagging change when I gain the ability to vote to close?
Unfortunately, since there are too few 20k users, relying on the community to delete posts is not a realistic possibility here at Super User. This is an exception to the rule that moderation be handled by the community as much as possible, since the community as it stands is unable to do so without ♦moderators.