I cam understand everyone's desire for perfection of course. We all want that.
But there comes a point when you have to make a judgement about the severity of an issue and whether its worth getting it replaced. Not because of the hassle of doing so (albeit, it IS a hassle), but because the replacement might well be worse.
I am on my 2nd 2012 iMac and if I took the "perfectionist" approach, I would be on my 3rd at least. Probably my 4th, 5th or 6th I reckon.
The first one had a spec of dirt behind the screen and visible clouding and screen non-uniformity. No brainer - replacement required. Having said that, it was in all other respects perfect.
The 2nd one leans to the left (the stand is not straight). And the fan sometimes makes a faint "helicopter" sound, rather than a smooth rush of air - quite offputting actually. So I thought about RMA'ing it as well. Then I figured its just not worth it. The "lean" is fixed with two business cards under the left side of the screen. And the fan doesn't do it all the time, and who knows, may sort itself out. If not, I can put up with it, or not, and have the fan replaced.
I had NO confidence that a 3rd iMac would be perfect.
If I were the OP, and everything else about the machine was OK, I would just forget about it.