Yeah as baddox says, you'd still be smart to vote 0% or 100%, as long as you can make a better than random pick at who is the leader and runner up. That's because voting in "black and white" is the same as Approval, and as I said, Approval gives an advantage to those who can guess better who are the leaders. Making it especially annoying in elections (such as local ones) where you have no idea who the front runners are.

It would depend on whether X or Y is more likely to win the election (according to polls). You can hurt X by voting 100% on Y.

Your specific vote is only important when the election is to close to call. If X is up by 80% in the polls you can probably stay home and nothing happens.