You can, but if you are converting between different aspect ratios it's a bit different, then it matters. Look at it this way, if you want to convert from a 4:3 monitor to a 16:9, the most consistent way is to keep the 4:3 portion of the 16:9 monitor the same as on the 4:3 monitor, so you just add a bit of extra width, like this:
This is a personal preference, but most aspects of the view and sensitivity are a bit more consistent by doing it like this.