Wikipedia agrees with mk: In a two-round election, I guess you can always vote for your favorite in the first round and do a tactical vote in the second. Protest vote seems to have its advantages, though they seem long-term: (1) It will increase recognition of the party; (2) It will increase the respectability of the party; (3) It will increase the recognized viability of the party. I voted blank this round. I don't think I'll be doing it again.For example, in a simple plurality election, a voter might sometimes gain a "better" outcome by voting for a less preferred but more generally popular candidate.
Tactical voting makes sense in the first round already, as the French experienced during the presidential election in 2002: many people voted for their favourite in the first round, so many in fact that the traditional incumbent didn't make it to the second round ; leaving a choice between the corrupt and lazy president versus an old racist...