I agree that you can do it on 28s. However, when my wife and I did the trail last fall, I switched her to some 41s (I think) specifically for the trip. She loves them and won't go back to her narrower tires, even on pavement rides. We were credit-card touring and only carrying clothing and a few snacks. If you are camping, the extra weight will further alter the ride quality and possibly make you appreciate wider tires.

If you do a lot of other riding, and always find the 28s to be sufficient, then go for it, but it might be worthwhile to at least try a wider tire and see how you feel about it. Wouldn't be worth getting new tires for a 150 mile ride, but if you liked them enough to keep riding them, it would be worth it.