Some county elections offices do and some don't make specific counts of "undervotes," which is the difference between the number of valid ballots counted and the number of votes in a particular race - the number of voters who bypassed a particular race. The undervote most commonly gets larger - which is to say, the vote gets smaller - as you move down a ballot. In presidential years, the undervote for president is usually microscopic, while the undervote for sewer district commissioner might be substantial.
The Spokane Spokesman-Review site has a provocative post on the undervote in last month's primary election - Republican division - for the office of sheriff. You might usually expect the undervote there to run rather high. In this case, the race was heated - signs and billsboards and letters to the editor about both candidates, Ozzie Knezovich and Cal Walker, were visible all over - so that logically would diminish the undervote. (Knezovich recently had been appointed sheriff, and was facing in the primary an incumbent Spokane Valley police chief. He is considered highly likely to win the general election.) But the Spokesman's Jim Camden checked the numbers and found it was unusually small, only about 3%. By comparison, the undervote in the U.S. Senate primary was about 10% (a fairly normal level for such a race).
Camden also, however, noted something else:
"The Democratic sheriff’s primary has the largest drop-off of voters, with nearly one in four not voting for newcomer James Flavel, or writing in someone else’s name. That’s more undervotes, by far, than any other Democratic primary in the county. What this suggests is that voters who took Republican ballots just for the sheriff’s race are either Democrats who are likely to shift back to their candidates in November, or independents who aren’t yet sold on the rest of the GOP ticket."
Another piece of the puzzle, as Spokane County works out what it will do when the parties compete on November 7.