Since 3D for new TV's is more or less dead it is a non issue for the future. I always render forced subs into video so no positioning problems
The information of having a 21:9 AR is important if you have an automatic screen masking when using a projector e.g. Otherwise of course no need to crop it.
In terms of encoding speed or getting smaller file sizes the black bars are really not making any sort of difference worth mentioning in my view.