I think that all depends on the reasons why you don't think you're right for it .
If it's an age or a cultural thing, usually, I'll email the director and ask them what they think. Directors are often very responsive as they don't want their time wasted either !
If it's because you think it's out of your range as an actor , you should still do it because you can learn a lot by stretching and sometimes you can change director's minds about the direction they were going for with a character.
My 02 cents.