Wait a moment, I have it,

Now when you take a key, say G Major, which has one sharp in it, you right the key signature at the start right?
Then, when you write in a note that isn't in the key of G major, you put an "accidental" next to it, such as a sharp or flat sign.

So basically, as far as I can tell, the only times you have to put in an accidental sign, is when you have a note that isn't in the key signature. Of course, if you have a piece of music that does not have a key signature, it is different.

So while Jolly and Cryptic are technically right, axemaster isn't totally wrong.