Modern LCD monitors should be positioned directly in front of you so you are viewing the screen dead ahead, not at angle on the right or left. Viewing LCD monitors at an angle can distort how you see the colours & the brightness level of the image (both those effects are especially to be avoided for photo-editing/printing of which I do a lot).
As for the correct distance, well that really depends on your own particular eyesight, but I imaging for a 24" screen it needs to be approximately 28" from your face. My screen is 19" and I have it 26" away which is just right for me as I'm near-sighted anyway, I wouldn't want it any closer.
I don't think there is a "recommended" distance, at least not in any documentation I've seen, it's a personal preference thing, whatever distance you are comfortable with.