I'm a big believer in this. I called it "let them marinate". It works particularly well in the modern world of IMs, where a lot of people see a problem and go "oh, X will know why this is happening, let's ask him!" without even trying to get their brain-gears moving. After a while, you learn who you should ignore and for how long, and it reduces the incentive for the worst time-wasters to come to you. I used to worry that they'd find me less useful and hence stop liking me, but that doesn't really happen.