Do everything in your power to get rid of chaff as fast as possible
Downvotes are somewhat less useful, especially if it's a 1-rep user or an unambiguously bad post1, but I still encourage you to do it, and absolutely flag and vote-to-delete (if you have that privilege). It only takes three delete votes from 20k+ users to get rid of a post, so the sooner you cast a vote the sooner it's gone.
The main moderation benefit to downvoting, as a 20k+ user, is that it allows you to immediately vote to delete answers2; otherwise you have to wait for it to show up in the Low Quality Posts review queue. So, for answers at least, that would be a compelling reason to downvote.
Flag spam or abuse, but don't VTD
Spam and abuse flags are slightly special in that enough of them, from users of any reputation level (with the privilege to flag, obviously) will automatically delete the post. It goes without saying that, if the flag is warranted, this is the fastest way to get rid of it.
However, it's probably not a good idea to vote-to-delete these posts, because posts that get auto-deleted through this mechanism are automatically locked, so it can't be edited or nominated for undeletion. Although this is a bit of an edge case, it does help mitigate the risk of disgruntled 10k+ or 20k+ user(s) undeleting a bunch of old garbage. I'm not entirely sure if that still happens if the post is deleted by user votes, but personally I'd rather not risk it.
I should mention that there might be an another reason not to vote-to-delete something that you've flagged as spam or abuse. Spam/abuse flags result in certain data being fed into some behind-the-scenes tooling, but it's not clear whether that happens only if the post is auto-deleted (as a result of six spam flags/one moderator spam flag) or if it just requires a valid spam flag (you can see Rand al'Thor and I discussing this in comments).
1 I should clarify what I mean here. Downvotes are the community's way of expressing displeasure with a post; it accomplishes this in four ways:
- It "punishes" (in the psychological sense) the user who posted the material by imposing a reputation penalty
- Through the visible score, it signposts to other users that this is undesirable content
- In some cases, it actively hides the post from view; questions with a score of -5 or lower are hidden from the front page, and answers with a score of -3 or lower are displayed in a faded, low-contrast colour
- Question/answer score is one of the factors that goes into temporary posting bans
These are all valuable things, so don't take this as me telling you not to bother voting. However, I would argue that the benefits of downvoting are diminished when it comes to posts that are obviously going to be deleted; because the post is going to be completely removed from the view of most users, there's relatively little benefit in visually differentiating it from other posts, so points 2 and 3 above still apply, but the utility is lessened.
In the case of a post by a 1-rep user, points 2 and 3 still apply but 1 does not; 1-rep users receive no reputation penalties whatsoever.
Again, the benefits of downvoting aren't removed, just diminished.
2 Unless the post already had upvotes. This is an occasional problem, much as I wish it wasn't