Well, like everything in life, this depends.
If you're talking strictly knives, that is. Some throwing objects you want to rotate no matter what (saw blades, for example, so the bits dig into the target).
There are two schools of thought on this.
The first school makes an entire science out of rotating a knife during a throw. The object is to work out the exact ratio between your grip and the blade's center of gravity. Typically, you want one turn every three feet of throw...but this is again affected by the blade length, your grip, and so forth.
In this scenario, let's say you've worked out exactly how to hold a given knife. You should be able to execute a forward overhand throw and stick the point in at three feet, six feet, nine feet, etc. All multiples of three.
If you stand four feet or five feet from the target, you won't stick it: at four feet you'll strike it with the butt of the thrower and at five feet it will slap against the target flat, etc. In reality, it won't be three feet, but you will find (for a given knife) that even multiples of your range will work.

Benefit: as long as you know your weapon's flight characteristics, you'll stick it nearly every time and look very impressive.
:grumpy: Disadvantage: that's about all you can do with it.
The second school of thought is the zero-spin throw style. Here, you don't want the knife to spin, roll, yaw, or pitch...but travel like a dart or a spear directly into the target. It's a much harder throw to learn, but eliminate the need to calculate ratios or stand at exact distances.

Benefit: you can stick the knife from any distance from two feet back to twenty, if your aim is good enough.
:grumpy: Distadvantage: the knife so badly wants to tumble or roll in flight that this can be a time-consuming, target-chewing, frustratingly repetitive thing to learn.
This throw can be done overhand, which is among the easier ways (I have numerous episodes where I struggle with it).
It can also be done sidearm. Forum member Will P. seems to have this throw exemplified. I've had nothing but problems with it, but with his advice, I've been amazed to see how easy it can get. Check it out here. (
http://www.bladeforums.com/forums/showthread.php?t=329767)
It can be done underhand as well, just by whipping your hand in a "bowling" motion toward the target. This sticks with some power from about 5m/5y.
There's also what we call a side-snapping throw (think frisbee), which is good for saw blades and other circular objects.
Hope this starts you out. I seriously doubt I answered your question, so let's drill down a little bit and see if anyone here can help you.