Since site selection is a ranked preference ballot (like the Hugo final ballot), one can influence the vote even if your first choice doesn't win.
The second/third/etc choices can definitely make a difference; the 2015 site selection vote was a choice between Helsinki, Spokane, and Orlando. After Orlando was eliminated (having the fewest #1 votes), its ballots were redistributed to their #2 votes which put Spokane ahead of Helsinki despite the latter having more #1 votes.
So if you like Helsinki better than DC but DC better than the other two, vote Helsinki #1 and DC #2....
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The second/third/etc choices can definitely make a difference; the 2015 site selection vote was a choice between Helsinki, Spokane, and Orlando. After Orlando was eliminated (having the fewest #1 votes), its ballots were redistributed to their #2 votes which put Spokane ahead of Helsinki despite the latter having more #1 votes.
So if you like Helsinki better than DC but DC better than the other two, vote Helsinki #1 and DC #2....