Should we have it be that if a post is deleted, any flag on that post is marked helpful, even if it was declined by a moderator?
In other words, say I flag an answer, and the flag is declined by a moderator. Later, though, the post is deleted by the community. Should we have it so that my flag counts as helpful? Maybe only if the post hasn't been edited?
No.
Different flags are used for different purposes, and have different effects within the system. For instance, spam flags carry heavy reputation penalties and contribute to spam-blocking systems under the hood. "not an answer" and "very low quality" flags kick the post into the Low Quality Posts review queue. If these weren't the right flag to raise, they should be declined, even if the post is delete-worthy because of other causes.
Also, moderators have a canned decline reason for custom flags that should have been regular flags. If someone raises a custom flag instead of a "not an answer" flag, requiring a moderator to manually handle the flag instead of allowing the flag to be handled by the community through the review queue, that's an inappropriate use of the flag and should be declined even if the post was delete-worthy.
So, no; there are valid reasons for declining flags even if the post later goes on to be deleted, and declining flags allows users to learn how to raise flags properly and work within the system.
