It depends on your own tolerance/perfectionism. If you don't mind the thought of cycling through a bunch of samples to eventually arrive at the one with the best uniformity, go ahead and exchange it. If you can live with it, shrug off this issue; no TV is perfect.
Everything you say is right though: your issue sounds relatively minor, and chances are when you pick up another sample of the same model (or any LCD TV, for that matter) you'll notice some uniformity issue. I've reviewed hundreds of LCD and LED TVs over the years, most of them samples hand-picked by the manufacturer, and nearly every one had imperfect screen uniformity. It's inherent in the technology.