I have seen the lines on 40 of 42 rev C MBAs. I find that most people just don't understand what the lines look like. On some, the lines look like grains in the display going horizontally across. Mostly on white to gray backgrounds. Sometimes on only part of the display. Most of the minor to average defective displays the lines are obvious from four to twelve inches but not from normal distance of more like eighteen to twenty-four inches. Seriously affected units have extremely obvious lines that are visible from average viewing distances to even farther away. I have seen lines from ten feet away at the Apple Store.
I would say that half of all I have seen are minor to moderate and half are seriously affected. People that report "no lines" on multiple units usually never see the lines or do not know what they're looking for. Once someone can detect a slightly affected unit, noticing any defective unit becomes instantly obvious.
Needless to say, of ALL of the displays that I have seen where someone else has proclaimed "perfect, without lines" have been defective. Therefore, when you report "without lines" it should be conveyed whether you can detect a defective display... from what I have seen, 95% have lines. And ALL that were proclaimed to be free of lines.
So, have you seen the lines before??? Or how is it you received two in a row without lines? The odds of that, based upon what I have experienced/witnessed, are extremely unlikely.
At the same time, I would encourage people to NOT look for the lines and just be happy. Yet report that you refuse to identify the lines. Most slightly affected units are not bothersome from normal viewing distance. I would just prefer people not proclaim as being free of lines when from my experience they are very likely to be reporting incorrectly. And incorrect reporting is a problem.
Just leads to those bothered by the lines being disappointed when they think the issue is corrected, but in reality nothing has changed. Just poor reporting by those who do not want to see the lines. I definitely encourage people to not even want to see the lines; but then don't report one way or another as to the lines.