I have the 40ex620.. Mine doesn't exhibit any vertical banding, however when the screen is black I do notice light in the 4 corners of the tv...
First time owning an edge lit led, should I assume it's normal with these televisions?
I'm pretty sure it is, but some TVs are worse than others. If it doesn't bother you much then I wouldn't worry about it. It's more of an issue when that light spreads significantly, and the screen looks overly bright in areas and blotchy. I experienced some of this but my bigger concern was that it carried over to darker images, so it wasnt just visible on an all black screen.
I'm now on my 3rd EX621. The first one must have been defective panel as the last two did not suffer from the horrendously narrow viewing angles and blue tinge it had. They have however very distinct flashlight effect in the bottom corners have some clouding/screen uniformity the first one did not have.
By dull, do you really mean not bright enough? One usually compensates by raising the backlight to 9 or MAX. If that's still not bright enough for one's situation, then I can understand turning off the light sensor as it does seem to put an absolute cap on effective brightness that might be too dim for brightly lit areas.