I disagree with this statement. Even the developer of that site suggests that it's possible that it could damage your screen and he assumes zero responsibility. You're saying it won't damage the computer, that's a very broad statement. See, if someone's screen gets screwed nothing will happen to you because you can hide behind your computer and just disappear off Macrumors.
Think about this too, say that the test reproduces IR, image retention can be temporary for the most part (unless a test like this causes burn-in) and will need to be reproduced, what makes you think that Apple will accept this test if a customer comes into the store and uses it to reproduce IR? They won't. A warning message like that will be taken seriously by Apple because it poses high risk.
You think you're helping people here, you're really asking them to do something risky on their expensive iMac.
I understand what you are saying 100%... and there are other ways to test for IR.
1. Set your background to dark gray (Preferences>Desktop&Screen Saver>Solid colors> and look for the dark gray at middle top row of solid colors
2. Pull up safari and go to your fav website and leave it up on your screen for 10 minutes (example, you can leave anything static on your screen)
3. minimize Safari and see if you have IR
If my iMac can't handle looking at the same image for 10 minutes without having IR, then I don't want it. The iMacs and Thunderbolt displays I have seen can handle the test easily. The point is that I have static images on my screen for more than 10 minutes when I am in Illustrator or Photophop. My samsung screens can handle it, my macbook air can handle it and so on and so forth.
If you are worried about what your iMac can handle, and think it's only designed for casual use... then I respect that and don't bother testing your LCD. If you have a faulty LCD, you might never know unless it gets so bad that you can't miss it.
On the other hand, many of us buy the iMac to do professional work on and we know the LCD is one of the best on the market and can handle black and white box images for 5 to 10 minutes or a picture from my last trip equally.
I tried the test for 3 minutes on a MacBook Pro Retina for 3 minutes and it had IR. It doesn't take 10 minutes to show IR, it only takes 2 or 3 minutes in most cases. I tried the test for 10 minutes on MBPr that has a Samsung screen and no IR. I regularly go to Apple stores and run the test. The only time I have seen IR is on the LG based MacBook Pro retina. That is the reason i did not buy it.
So again, please test your LCD in a way that you are comfortable with. The test that I recommended has been used by thousands of people on this and the official support forum. Not a single person has said it damaged their computer, and if you know enough about LCDs, you will know that looking at the same image for 10 minutes shouldn't be an issue. Otherwise, follow my instructions above for a more real world test. Or, come up with your own test after you read more about IR on the various support forums.
Here is another test, and you can set the timing:
http://www.mitchmalone.name/retina-test/