Can I just make wall openings the desired width to fit the door or window?
This is not recommended. Remember that top row of lintel blocks that are required for your bond-beam? If you fudge on the door opening, then the block rows above the door opening will not fit because the wall segment including that door has to remain the same at the top and bottom. If the door opening does not follow the dimensioning tables then the block rows above the opening will have different lengths than the block rows below the opening. This means the top of your wall would be different than the bottom of the same wall unless you cut some blocks above the opening.