When brightness settings etc have been correctly adjusted, I shouldn't think that ANY burn-in will be experienced, -
in average/normal use.
Tweaking brightness settings is vital, IMO. - Out of the box, it is MUCH too bright! - It will hurt your eyes....
as well as your LCD screen! 
That depends on what you consider to be "average/normal use". Not to be contrary here, but the difference in ambient lighting between the average home office/den, and an office cubicle under multiple flourescents, can make a huge difference in what brightness level is desirable. And then there is the matter of lots of novice computer users who never realize that there even ARE brightness controls on their monitors. The vast majority of customers I run across have never done any sort of adjusting on their LCD's. They just pull them out of the box, plug them in, turn them on, watch the screen say something about "auto adjusting", and then never touch anything other than the power button from there on out.