Even if it is NP Complete, that would really just limit you on the size of a text for which you could find the absolute best line breaking.
Consider, the traveling salesman for the capitals of the US is easily solvable, due to the small size of the problem space. Similarly, I'd imagine the actual problem space to not be that large on line breaking.
Specifically, since line breaking is limited to the paragraphs you are looking at, this probably isn't an issue in most cases. Degenerate cases could be checked for and processing limited to not try too much.
Consider, the traveling salesman for the capitals of the US is easily solvable, due to the small size of the problem space. Similarly, I'd imagine the actual problem space to not be that large on line breaking.
Specifically, since line breaking is limited to the paragraphs you are looking at, this probably isn't an issue in most cases. Degenerate cases could be checked for and processing limited to not try too much.