Guys, just remember a car on a dyno does not move. ...
+1. This is why final tuning is done on the track. However (and this plays directly into all the testing done on the dyno), the best values you find while testing are still the target when you hit the track. When you road the car, stuff will change as Billy says, and that's when you use the dyno results to know how and where to correct for the changes.
For example, the jetting does change and differently in every car, but you know where you need to correct it. You are still after the best AFR found, and now you have to make changes for fuel distribution, with stuff like staggered jetting, fuel tabs, throat mods, etc. Or, the jetting may go lean only at launch, and changes in your pump shot may be better than the overall jetting, that may quickly recover after launch.
Except for changes in weather or track/suspension condition, your tune should be stabilized at that point. Likewise, those new condition changes would mean only a slight adjustment of your now stable tuning, only compensating for the effects.
This is one reason racers keep all the dyno sheets, so they know from testing what change in tuning will cause what effect, such as how much the AFR will recover on a hot or cold day with one step jet change. Just one more reason EFI has become so popular, as it reads all that, and (if set up properly) will correct it for you automatically. Those pesky sensors are what give the system (and you) the information to know what to do to maintain and even improve peak performance.
[EDIT] - oops, I forgot this comment:
Only full throttle runs will reveal the proper jetting.
And to be clear - that's entirely true for a drag-only car. If it's an auto-x or other variable-throttle racer or street car, then for best performance in all throttle and load ranges (including WOT), then full throttle runs will reveal the best power AFR. Then the trick is to set best jetting for midrange, and combine it with tuning the power valve and PVCRs, to finally achieve best AFR and power at WOT. Tuning drag cars is easy. Tuning variable-throttle cars is a whole different planet. Note my comments above about part-throttle sweeps, and inertial or steady-state dyno types.