Thanks Airshot. I can see how long side bunks would really center the boat, especially if driving the boat up on the trailer. They fit the boat kind of like a glove. With guide posts the bow of the boat could possibly go off center depending on the set-up. But once you snug up the bow winch, the rear of the boat would be centered and settle right down on the bunks. At least that is my experience.
Either way, your post made me realize I should seriously consider the option of another set of brackets and longer side bunks. In my case the current set-up works fairly well, so it is really one of those "nice to do" as opposed to "must do" things. The main issue it would avoid is what
@Boosh mentioned -- on some ramps the boat can float over the short side bunks at the rear of the trailer.
By the way, even with my short ones, I unbolt and remove the side bunks when cleaning the boat (probably not as often as I should).