TL;DR: The answer is yes. You don’t have to rank all the candidates, but there’s no reason not to.
The OpenStreetMap Foundation (OSMF) is currently holding an election for four seats on their Board of Directors. This is the governing body for the global OpenStreetMap project, and ideally the Board will represent all the diverse perspectives and communities within the overall OpenStreetMap movement. Thankfully, the OSMF board uses the Single Transferable Vote (STV) method for its elections (also known as multi-winner Ranked Choice Voting), which is perhaps the most robust and flexible form of Proportional Representation, giving the voting public the chance to elect representatives that fairly reflect the diversity of their views, without requiring candidates to form political parties or requiring that the voting public be divided up into artificial geographic regions. I previously wrote a post explaining the benefits of STV for OSM elections here.
As someone who has administered several STV elections in the past (as the elections observer for the OSM-US board and as a co-founder of FairVote Washington) a lot of people ask me for advice about how to fill out their ballot. Not about who to vote for, but how to fill out their rankings so that the candidates they like have the best chance of winning. Specifically, they most often ask whether they should fill out all the rankings, or leave some candidates unranked. Imagine that there are 12 candidates running, and you like six of them and dislike the other six. Should you rank your top six in order and leave your other preferences blank? Or should you keep going and rank your disliked candidates from 7 down to 12, your absolute most disliked candidate?
The great thing about Ranked Choice Voting (especially in a multi-winner form like STV) is that there is practically no reason to ever vote strategically. You can sincerely rank all the candidates in order of preference, from your most preferred all the way down to your least preferred, and be confident that none of your rankings will harm the chances of your favorite candidates to win. Simply put: in an STV election you can and should rank all the candidates according to how much you like or dislike them.
I have heard several people say that you should only rank candidates you like, and don’t rank any candidates you don’t want to win. This is a misconception. Under many non-STV systems, this is a good strategy. But with STV the only reason not to provide rankings for candidates you dislike is if you legitimately dislike them all equally. If you really truly don’t care which of the bad candidates might win, then you can leave those bottom rankings blank. But in my experience, most people do have some opinions about which candidates they somewhat dislike versus those that they really really would hate to see win. If you have these opinions, it’s okay to express them on your ballot. To emphasize it again: providing rankings for your disliked candidates will never harm the candidates you truly like.
We can see how this is true by examining how votes are tallied in an STV election. The important thing to remember is that you only have one vote, even though you have many rankings that express your instructions about how your vote should be allocated. Hence the “Single” in “Single Transferable Vote”. As the votes are counted in rounds (where some candidates in each round are elected if they’re above the threshold, or some candidates are eliminated when they’re on the bottom of the stack) your vote always stays with your highest-ranked candidate who is still in the running at each round. The only way your vote would ever transfer to one of your disliked candidates is if all of your more preferred candidates have already been elected or already been eliminated. So the only time where your vote would start counting for your disliked candidates is when your disliked candidates are the only ones left in the running. No matter what you do, the election is over for your favorite candidates. At this point, wouldn’t you still want to have some say about which of the disliked candidates goes on to win? If you ranked all the candidates, you would still have some say to help elect the lesser of these evils. If you left your lower rankings blank, then you would be sitting out these final rounds of the tally.
Here’s an article from New Zealand describing STV voting strategy in civic elections, which also concludes that there’s no harm in ranking candidates you dislike: https://www.odt.co.nz/opinion/stv-voting-strategy-candidates-you-dislike
Finally, however, I want to reiterate that you don’t have to rank all the candidates. If you truly do not have an opinion about some names on your ballot, it’s fine to leave them unranked. This will not make your ballot invalid, and it will not help nor hurt your favorite candidates. If you feel like ranking all the candidates is overwhelming and might deter you from voting at all, don’t worry about it! Even if you only rank one candidate, that’s still better than not voting at all!
The point of this post is not to stress you out and make you feel like you should rank everyone. I merely want to drive home the point that you shouldn’t hold back for fear of hurting your favorites.