Letterboxing can give levels a more cinematic look, and it can also be used to make levels harder by lowering the amount of visible space (consider how much harder jj1 is than jj2 - its because of the resolution). However, you have to design your levels around it, and it also limits how many layers you can use. And a last point to consider is that some players find it annoying.

I personally recommend going without it, but its really up to you what style you go for.

Oh yeah, and another thing you can use that for is a sort of GUI - think of what Tomb Rabbit 1 had. You can do fancier stuff these days since it's possible to trigger stuff in the foreground (use Reworder). Think of the text box you had in that example level, but actually useful. You don't need to use a black bar for this though - it would work just as well to have a small unobstructive box at the side of the screen. I notice that you are going for a style where the level tells you when you find a secret area; perhaps you could add triggers to that, and make something pop up in the foreground? Like each time you find a secret, a thing lights up, and if you light up all of them you know you've found them all (and maybe get a bonus at the end of the level or something). This is more interesting and more useful than text.