Do this. Set the brightness to what you normally run at, load a video or movie that is letterbox, and watch it. If the bleed is distracting, that would be bad, and by distracting, I mean it it so blatantly obvious that your eyes gravitate to it even if it's a really good movie.
Now, load up another movie that is full screen. Does the bleed affect the colors of the video? If so, take it back.
If it's not bad and you're just worried about it getting worse, all I can tell you is that I have had my iPhone for over a year now, and the bleed is the same as the day I got it. I believe I can get worse if you run your brightness high all the time, but even then, I think you won't have the phone long enough to see that happen.