If a challenge is tagged with code-golf or fastest-code, do you really have to say that that is the winning criteria in the question? It seems pretty obvious, just by looking at the tags, right? After all, we don't say "this is a discussion" on questions tagged with discussion, do we?
However (just playing devil's advocate here), if you don't explicitly state how to chose a winner, people may not notice, leading to a whole lot of confusion as to why someone's post has been closed and did people not actually notice this and so on and so forth.
If a question looks something like this
Easy Prime Question
Write a program that outputs a prime number.
Does the obligatory "Shortest code wins" really have to be included, along with the code golf tag?